I've only ever played a handful of characters over the fifteen years or so I've veen RPing. There's a rare instance (mostly recently while trying to find a MUSH to play on...lots of turn around in the last six months) where I don't play a character for at least six months, and even then that's kind of short, for me.
What I find works the best (for me):
*It's been mentioned, but not detailing a background too much - or at least not having the most interesting things in their life, in the background.
*Making a concept that is flexible. Have your specific quirks and hooks that you stick to (because you liked the character enough to make it, stick with those reasons why), but try not to have them dictate every interaction. Influence, sure. But if you know what's going to happen in every scene because of a certain flaw, or whatever, then it's going to get boring.
*Roll with the punches. (Figuratively and literally). Figuratively - cannot stress being flexible, enough. Literally - don't be afraid of IC confrontation. Don't be afraid of short scenes. Short scenes can easily lead to more plot hooks. Characters not getting along at first, can lead to more plot hooks (assuming the person ooc wants to rp again).
*Curiosity - it kills the cat, but doesn't tend to kill plot unless you're a real jerk about it. This ties into "drop the pencil", a lot. Yes, drop the pencil, but try to respond when others do, too.
Aaand, that's all I can think of specifically, right now. Good luck! And RL willing, if you (or anyone) is on Fallcoast and doesn't mind a Changeling, I'm Red over there. Feel free to poke me for scenes! I'm not always available when on (i idle-chat at work a lot), but I do love meeting new folks
ETA: Cliques? Forget cliques. I have my prefered rp people, but I feel like there's always one clique on a mu*, and then there's everyone else who thinks everyone else is in a clique, so they get frustrated and don't branch out. When everyone does that, that makes for a lot of lonely RPers. Branch out! The more people you meet, the more plot hooks you have.
I went to The Reach all by my lonesome (mostly because who dragged me there totally bailed. Not cool, man =p), and managed to find people to rp with, despite being overwhelmed and super shy, because I took that first step of branching out. Don't be afraid to help others with their characters, either. Often they'll return the favor. I'll stop rambling now =D