For me?
- Study the shit out of the market board. Look for items over a 3-4 day period that consistently sell. Any good step towards making money in any MMO is to get a feel for the overall economy.
- Crafting materials (lumber, ingots, threads, cloth, etc.) are always a valuable resource and will always be in demand so paying attention to those are a good start!
- Level at least three crafting classes. My recommendation? CRP, GSM, and BSM. LWR, WVR, ARM is another one too for some.
- Dont put up more then 6 or 7 listings of an item at a time. Because more often then not if they don't sell quickly you'll have a bunch of people undercutting you and you may not get your money's worth. Dabble in a few places and move along.
- Check the MB regularly for differences in trends.
- Do some research. Google is your friend! Take a peek at items that may be hard to come by, find a way to get them, and GO GO GO!
- Exploit the fact you can level up your retainers to get stuff for you. Especially have at least one maxed combat and one maxed gathering class for use.