A lot of people have covered the things that I would say: be prepared, have fun, help others have fun, bring beer or caffeine, a functional character sheet that prompts the player on how rules typical to their character work, etc.
So, what I'll say is that a basic requirement is to have a character that's reasonably functional. Every player in the party is depending on other players to do certain jobs well so that they can have fun. It's not fun if your character is constantly dying and it's not even the result of your decisions. If you're the PC who's going to be the face guy, devote some resources to being good at that task: items, abilities, feats, skills. If you're the tank, maybe light armor's not such a good idea. And maybe the party doesn't need a third skill monkey. If the party needs a cleric, the party needs a cleric. Try to meet the rest of the players AT LEAST halfway in your character design. It would be nice if that character concept also met the DM's campaign concept halfway as well.
In short, communicate with other players and build a solid character mechanically, so that everyone can have fun.