I see a lot of general advice in this thread so far (which is good! especially if you're being overgeared, which ain't fun!) but not many actual tips for WAR tanking. WAR-NING: likely to be more of a guide. Fuggit, let's go.
Golden Rule of WAR (may also apply to PLDs, refer to others on that): Given similar gear levels, a WAR should never lose aggro once a hate/enmity lead has established; if aggro is lost, the WAR has done something wrong.
The reason this is the case is because, as a WAR, you have many good tools (and in the case of Overpower and Steel Cyclone, incontrovertibly superior tools) for establishing, maintaining, and building said lead.
Something to understand about enmity: it's a race where early leads count for far more than you'd expect them to. That's why the rule of thumb is, after pulling with Hawk/Provoke/whatever, to either Steel Cyclone or Overpower at least two to three times. That's why you see bad WARs getting by just by spamming Overpower on trash pulls:
1-2-3 Butcher's Block Combo: Standard fare, nothing special. Standard tank M.O. is to tab-target/cycle target through the enemy list. Heavy Swing just delays the inevitable, Skull Sunder keeps you even most of the time if not pulling ahead, and Butcher's Block just gives you a huge boost in enmity on your target thereby letting you pull ahead.
Overpower: A WAR's bread and butter for establishing, building, and maintaining aggro on multiple targets. Frontal AoE cone, but it'll run you dry soon... unless you have a Bard singing Army's Paeon for you, in which case you can pull entire dungeons between bosses without pausing for TP. Pop Unchained or Berserk and it hits harder. Again, rule of thumb: Overpower twice or three times when you're getting started on a trash pull or when picking up bunched-up adds. Throw one in every once in a while between combos if needed.
Flash: Flash deserves a mention because, while we don't get the Blind and Bind bonuses that PLDs get, Flash is on an entirely different resource than TP. Flash uses MP. WARs don't use MP for ANYTHING ELSE. This means you have three or four Flashes to mix in whenever you need them to hold aggro (you won't be building aggro with this). Benefits: using it mid-combo doesn't break the combo, and using it on sleeping enemies doesn't wake them up, meaning you can beat your healer down the enmity list so that sleeping enemies don't go for them when they wake up.  Flash has enough utility for WAR that it's worth crossclassing for most content that isn't Insert-Primal-Fight-Here.
Maim: The Storm's Path / Storm's Eye Combos: Infinitely better combos than their contemporary, Riot Blade, on PLD. Maim is already great, and ideally you want to keep that buff up on yourself at all times: the more damage that goes into your attacks, the more enmity you're building. Storm's Eye follows the same concept: applies a debuff to a single target that weakens their resistance to slash damage, meaning you and PLDs hit them harder... which means you're doing more damage, which means more enmity being built. Storm's Path is great too, single debuff on a target that keeps them from hitting as hard and you can keep 100% uptime going on a boss if you know what you're doing. So. One self-buff in Maim, and two debuffs in the Storm combos. USE THEM. They're part of what gives WAR its edge.
Defiance and Wrath: This mechanic is fairly easy to understand. Defiance is a no-brainer: you gain more max health, healers get a multiplier when healing you, and in exchange for losing DPS you gain an enmity multiplier than more than makes up for the loss from downgraded damage. Wrath is simple enough too: the more stacks you have, the more often you crit, and therefore the more damage you do more often. Hit five stacks of Wrath, and you can pop Unchained to remove your DPS limiter (yay~! base damage AND enmity multiplier!), Inner Beast for a hard-hitting self-healing attack that gives you some serious Damage Reduction, or Steel Cyclone for ENMITYYYYYY.
Wrath needs to be built, however. Skull Sunder, Butcher's Block, Maim, Storm's Path, Storm's Eye, Berserk, and Vengeance all give one stack per use. Infuriate instantly puts you at five stacks.
Steel Cyclone: You hit everything around you with some serious potency. It's an AoE that gets an enmity bonus. AND ignores the Defiance DPS penalty. The best part? Once you're 50, you can pop Infuriate to give yourself an instant 5 stacks of Wrath... meaning that you can use Steel Cyclone at will so long as you have Infuriate. You can lead with this, build up to it by target-cycling your BB combo, or double-cast it by building up to 5 stacks of Wrath, popping Cyclone, then using Infuriate and using Cyclone AGAIN. Â
Fracture: Only truly worth it on a MRD/WAR, since you get 30s duration. It counts as damage, which means it contributes ever so slightly to enmity. It's not a priority, but every little bit can help (WARNING: DON'T USE THIS FOR ENMITY PURPOSES. IT'S A DAMAGE SKILL, USE IT FOR DAMAGE. A LITTLE MORE ENMITY IS JUST ICING ON THE CAKE).
Unchained and Berserk: The real reasons a WAR should NEVER lose aggro to a near-equally-geared DPS. As previously mentioned, Unchained removes the 25% DPS penalty from Defiance for a short while, giving you your DPS back on top of your now-boosted enmity. Berserk just gives you boosted attack power. Couple these together (and maybe pop Internal Release if you've cross-classed it), start going into Butcher's Block combos (get Storm's Eye up before popping Unchained/Berserk if you can), build back up to Wrath V for the increased crit chance and suddenly you gain enmity faster than anything else in the game bar a Medica II -spamming White Mage.
Silver Rule of WAR: PLDs pop a CD (cooldown) or two and they get the listed effects; WARs mix and match their CDs to achieve a desired effect, whether that be MASSIVE ENMITY BUILDING or HAHAHAHA I AM PRACTICALLY INVINCIBLE FOR 15 SECONDS.
Example: Vengeance not only gives you 30% DR (damage reduction) and a Wrath stack, it also redirects a portion of incoming physical damage (read: autoattacks, mostly) back at the source. That means if a bunch of mobs are wailing on you and you pop Vengeance, you are suddenly effectively wailing back on the mobs. So... have a large group of trash? Couple Vengeance with, yup, Bloodbath and suddenly... you're mitigating their damage more than 30% because you're effectively healing off the damage you're dealing to each and every mob.
Now... throw Unchained + Berserk + Overpower (another nifty combination) into the mix, and suddenly you're healing THAT MUCH MORE. Pop a cross-classed Second Wind while you're at it and you've got a large heal (yes, Berserk ups the heal on SW by upping your attack power) at your beck and call. Â
So. Optional in parans.
(Infuriate) -> Bloodbath -> Start Overpowering -> Vengeance -> (Unchained) -> Berserk -> (Second Wind) -> OP, OP, OP until Bloodbath runs out. Add defensive CDs like Foresight when and where appropriate.
Inner Beast: Hard-hitting 300 potency that ignores the 25% dps penalty from Defiance, heals for 100% of damage dealt, and gives you 20% damage reduction for 10 seconds. Coupled with Storm's Path and Vengeance? Holyyyyy sheeeeeet son, you're, like, the Juggernaut. Best part? Cycling through your BB and Storm combos, you can have Inner Beast ready for use roughly every 18 seconds, discounting Infuriate. This thing is core. This thing is why hardcore PvE folks take WARs as Main Tanks against hard-hitting content like Twintania's Death Sentence. With Storm's Path, every 18 seconds you have 30% Damage Reduction for 6 seconds. That's roughly 18 seconds of 30% DR for every minute-and-six-seconds. Start drooling.
Holmgang: So. Bloodbath lets us RIP AND TEAR and eat huge guts to get healthy again (WAR is kinda about managing HP gain/loss compared to PLD's preventing HP loss). Holmgang lets us take fatal punches to the face like the glorious berzerkers we are and keep right on trucking. Six seconds that you can't move. Six seconds that your target is bound in place. Six seconds that you can't BE moved (screw you, Landslide!). Six seconds of effective invincibility where you literally cannot go under 1 health point. You don't even need Benediction or spammed Lustrates from your healers to get you back up: done right, assuming you saved some CDs, you can pop stuff like Inner Beast, Foresight, Vengeance, Thrill of Battle, etc. to get yourself enough of a cushion to more or less COME BACK FROM THE DEAD. Hallowed Ground is pansy by comparison (we still <3 you, Pally friends).
Thrill of Battle: Ya wanna survive Meteor? This thing effectively boosts both current and max hp for a significant duration, meaning it also acts as a self-heal of sorts. Also acts as a health buffer if you use it while topped off. Not as exciting as everything else... but hey.
Mercy Stroke: Just as clutch as Holmgang. Last-hit something with this, and you get a significant chunk of health back as a self-heal. Couple with Unchained and Berserk if you need to hit REALLY hard to insure you get the last hit instead of your damage dealers. Off GCD Instant skill, meaning you can slip it between weaponskills... meaning you can snag some health off that trash that's about to die while target-cycling. If you're not worried about getting health back or you're vs. a boss, just cast this whenever it's up and your target is low on HP for a DPS increase.
I'm sure I've forgotten a ton of stuff I could be talking about... but that'll get you going. WAR's more complex IMO than its counterpart tank in PLD.
Golden Rule of WAR (may also apply to PLDs, refer to others on that): Given similar gear levels, a WAR should never lose aggro once a hate/enmity lead has established; if aggro is lost, the WAR has done something wrong.
The reason this is the case is because, as a WAR, you have many good tools (and in the case of Overpower and Steel Cyclone, incontrovertibly superior tools) for establishing, maintaining, and building said lead.
Something to understand about enmity: it's a race where early leads count for far more than you'd expect them to. That's why the rule of thumb is, after pulling with Hawk/Provoke/whatever, to either Steel Cyclone or Overpower at least two to three times. That's why you see bad WARs getting by just by spamming Overpower on trash pulls:
1-2-3 Butcher's Block Combo: Standard fare, nothing special. Standard tank M.O. is to tab-target/cycle target through the enemy list. Heavy Swing just delays the inevitable, Skull Sunder keeps you even most of the time if not pulling ahead, and Butcher's Block just gives you a huge boost in enmity on your target thereby letting you pull ahead.
Overpower: A WAR's bread and butter for establishing, building, and maintaining aggro on multiple targets. Frontal AoE cone, but it'll run you dry soon... unless you have a Bard singing Army's Paeon for you, in which case you can pull entire dungeons between bosses without pausing for TP. Pop Unchained or Berserk and it hits harder. Again, rule of thumb: Overpower twice or three times when you're getting started on a trash pull or when picking up bunched-up adds. Throw one in every once in a while between combos if needed.
Flash: Flash deserves a mention because, while we don't get the Blind and Bind bonuses that PLDs get, Flash is on an entirely different resource than TP. Flash uses MP. WARs don't use MP for ANYTHING ELSE. This means you have three or four Flashes to mix in whenever you need them to hold aggro (you won't be building aggro with this). Benefits: using it mid-combo doesn't break the combo, and using it on sleeping enemies doesn't wake them up, meaning you can beat your healer down the enmity list so that sleeping enemies don't go for them when they wake up.  Flash has enough utility for WAR that it's worth crossclassing for most content that isn't Insert-Primal-Fight-Here.
Maim: The Storm's Path / Storm's Eye Combos: Infinitely better combos than their contemporary, Riot Blade, on PLD. Maim is already great, and ideally you want to keep that buff up on yourself at all times: the more damage that goes into your attacks, the more enmity you're building. Storm's Eye follows the same concept: applies a debuff to a single target that weakens their resistance to slash damage, meaning you and PLDs hit them harder... which means you're doing more damage, which means more enmity being built. Storm's Path is great too, single debuff on a target that keeps them from hitting as hard and you can keep 100% uptime going on a boss if you know what you're doing. So. One self-buff in Maim, and two debuffs in the Storm combos. USE THEM. They're part of what gives WAR its edge.
Defiance and Wrath: This mechanic is fairly easy to understand. Defiance is a no-brainer: you gain more max health, healers get a multiplier when healing you, and in exchange for losing DPS you gain an enmity multiplier than more than makes up for the loss from downgraded damage. Wrath is simple enough too: the more stacks you have, the more often you crit, and therefore the more damage you do more often. Hit five stacks of Wrath, and you can pop Unchained to remove your DPS limiter (yay~! base damage AND enmity multiplier!), Inner Beast for a hard-hitting self-healing attack that gives you some serious Damage Reduction, or Steel Cyclone for ENMITYYYYYY.
Wrath needs to be built, however. Skull Sunder, Butcher's Block, Maim, Storm's Path, Storm's Eye, Berserk, and Vengeance all give one stack per use. Infuriate instantly puts you at five stacks.
Steel Cyclone: You hit everything around you with some serious potency. It's an AoE that gets an enmity bonus. AND ignores the Defiance DPS penalty. The best part? Once you're 50, you can pop Infuriate to give yourself an instant 5 stacks of Wrath... meaning that you can use Steel Cyclone at will so long as you have Infuriate. You can lead with this, build up to it by target-cycling your BB combo, or double-cast it by building up to 5 stacks of Wrath, popping Cyclone, then using Infuriate and using Cyclone AGAIN. Â
Fracture: Only truly worth it on a MRD/WAR, since you get 30s duration. It counts as damage, which means it contributes ever so slightly to enmity. It's not a priority, but every little bit can help (WARNING: DON'T USE THIS FOR ENMITY PURPOSES. IT'S A DAMAGE SKILL, USE IT FOR DAMAGE. A LITTLE MORE ENMITY IS JUST ICING ON THE CAKE).
Unchained and Berserk: The real reasons a WAR should NEVER lose aggro to a near-equally-geared DPS. As previously mentioned, Unchained removes the 25% DPS penalty from Defiance for a short while, giving you your DPS back on top of your now-boosted enmity. Berserk just gives you boosted attack power. Couple these together (and maybe pop Internal Release if you've cross-classed it), start going into Butcher's Block combos (get Storm's Eye up before popping Unchained/Berserk if you can), build back up to Wrath V for the increased crit chance and suddenly you gain enmity faster than anything else in the game bar a Medica II -spamming White Mage.
Silver Rule of WAR: PLDs pop a CD (cooldown) or two and they get the listed effects; WARs mix and match their CDs to achieve a desired effect, whether that be MASSIVE ENMITY BUILDING or HAHAHAHA I AM PRACTICALLY INVINCIBLE FOR 15 SECONDS.
Example: Vengeance not only gives you 30% DR (damage reduction) and a Wrath stack, it also redirects a portion of incoming physical damage (read: autoattacks, mostly) back at the source. That means if a bunch of mobs are wailing on you and you pop Vengeance, you are suddenly effectively wailing back on the mobs. So... have a large group of trash? Couple Vengeance with, yup, Bloodbath and suddenly... you're mitigating their damage more than 30% because you're effectively healing off the damage you're dealing to each and every mob.
Now... throw Unchained + Berserk + Overpower (another nifty combination) into the mix, and suddenly you're healing THAT MUCH MORE. Pop a cross-classed Second Wind while you're at it and you've got a large heal (yes, Berserk ups the heal on SW by upping your attack power) at your beck and call. Â
So. Optional in parans.
(Infuriate) -> Bloodbath -> Start Overpowering -> Vengeance -> (Unchained) -> Berserk -> (Second Wind) -> OP, OP, OP until Bloodbath runs out. Add defensive CDs like Foresight when and where appropriate.
Inner Beast: Hard-hitting 300 potency that ignores the 25% dps penalty from Defiance, heals for 100% of damage dealt, and gives you 20% damage reduction for 10 seconds. Coupled with Storm's Path and Vengeance? Holyyyyy sheeeeeet son, you're, like, the Juggernaut. Best part? Cycling through your BB and Storm combos, you can have Inner Beast ready for use roughly every 18 seconds, discounting Infuriate. This thing is core. This thing is why hardcore PvE folks take WARs as Main Tanks against hard-hitting content like Twintania's Death Sentence. With Storm's Path, every 18 seconds you have 30% Damage Reduction for 6 seconds. That's roughly 18 seconds of 30% DR for every minute-and-six-seconds. Start drooling.
Holmgang: So. Bloodbath lets us RIP AND TEAR and eat huge guts to get healthy again (WAR is kinda about managing HP gain/loss compared to PLD's preventing HP loss). Holmgang lets us take fatal punches to the face like the glorious berzerkers we are and keep right on trucking. Six seconds that you can't move. Six seconds that your target is bound in place. Six seconds that you can't BE moved (screw you, Landslide!). Six seconds of effective invincibility where you literally cannot go under 1 health point. You don't even need Benediction or spammed Lustrates from your healers to get you back up: done right, assuming you saved some CDs, you can pop stuff like Inner Beast, Foresight, Vengeance, Thrill of Battle, etc. to get yourself enough of a cushion to more or less COME BACK FROM THE DEAD. Hallowed Ground is pansy by comparison (we still <3 you, Pally friends).
Thrill of Battle: Ya wanna survive Meteor? This thing effectively boosts both current and max hp for a significant duration, meaning it also acts as a self-heal of sorts. Also acts as a health buffer if you use it while topped off. Not as exciting as everything else... but hey.
Mercy Stroke: Just as clutch as Holmgang. Last-hit something with this, and you get a significant chunk of health back as a self-heal. Couple with Unchained and Berserk if you need to hit REALLY hard to insure you get the last hit instead of your damage dealers. Off GCD Instant skill, meaning you can slip it between weaponskills... meaning you can snag some health off that trash that's about to die while target-cycling. If you're not worried about getting health back or you're vs. a boss, just cast this whenever it's up and your target is low on HP for a DPS increase.
I'm sure I've forgotten a ton of stuff I could be talking about... but that'll get you going. WAR's more complex IMO than its counterpart tank in PLD.