Okay, I'm sure this one's been done before, but the various threads on bad player behavior and how to be a better DM have made me want to start this discussion. Here are my suggestions for being a better player (not necessarily in order of importance):
1. Be prepared. Always bring whatever you need to bring. Know what you need to bring. Even if the GM provides everything, be mentally prepared to play when you arrive.
2. Pay attention. Don't sleep, read, watch tv, kibbitz with other players, play your GameBoy, snog with your girlfriend/boyfriend, play with the puppy, or do anything else that keeps you from keeping your mind on the game (this one is my downfall, I fear).
3. Know what is expected of you, both in the game and out of the game. If you don't know, ask.
4. Show up on time, and if you can't, let everyone know as far in advance as possible.
5. Have fun. But not at the expense of everyone else's fun.
6. Treat everyone with respect.