I try to keep it to only being the stuff I can't avoid mixing in, because of the way my head is screwed together irl. What I do instead, and something I find really interesting, is that I carry over some traits/quirks/habits from my past longer-standing characters, which ends up with being rather interesting combinations. Of course not to the point of being an exact copy, but just in the way that the character makes it work - to other people who don't know my past characters, it wouldn't seem out of place or familiar to anything, but I find it a little funny to keep these aspects in the character.
