I would like to follow up with a shorter (well, maybe) related topic. Rules for players. That's right, players, you've got responsibilities too.
Rule 1: This is a group game
Everyone likes attention, or most gamers do, anyway. That's fine. But remember that this is a group activity. If you're trying to hog the limelight in every scene, you're probably ruining the game for everyone else. If that doesn't bother you, you're not the kind of person who should have friends.
That doesn't mean you shouldn't take charge when the situation warrants. If you're playing the half-orc fighter with the +3 greatsword, yeah, you're probably going to dominate combat. At the same time, don't assume that everyone else is useless, and don't expect every combat to focus on you.
Rule 2: You are not there to screw up the GM
That doesn't mean you can't throw a monkey wrench into his plants. Surprise him. Make decisions he didn't expect--so long as they're character-appropriate. Make him keep up with you.
But--don't make things deliberately hard on him. Don't play a character that's going to be trying to kill the rest of the party, unless the GM has approved that sort of thing. Don't play a character who's going to refuse to stay with the group; if the GM has to struggle to find reasons for you to be together, you're best served making a new character.
It's the GM's job to make sure that you're having fun; it's your job to make sure that he is.
Rule 3: There's a time for arguments
If you strongly disagree with a GM call, there's nothing wrong with letting him know--briefly. But if he still disagrees, the middle of the game is not the time to hash it out. Nothing ruins the game faster than watching one of your fellow players go head-to-head with the GM over whether or not that Tremere is low enough generation to use a 6th level Discipline, or whether the dragon should be able to cast that spell and attack in the same round. Go with it for now, and discuss it after the game. If he's reasonable, he'll at least hear you out. If he's not, why are you playing with him? (Yes, I'm guilty of this one myself. I'm working on it.)
Rule 4: What you know is not the same as what your character knows
It's amazing how often even experienced players forget this. Your character has not read the rulebook cover-to-cover, even if you have. Just because you know that trolls are vulnerable to fire, or that the Tome of Icky Badness costs you 2d6 Sanity points if you read it, that doesn't mean your character knows. If your character doesn't know something, you can't act on it. Period.
Rule 5: You're here to game
There's nothing wrong with friends just hanging out and chilling, talking about work or the baseball game or whatever. But that's not why you're here tonight. No sane GM is going to penalize you for the occasional OOC comment or joke--but keep them occasional. In the midst of a tense negotiation with the Nosferatu primogen or King Hounblat is not the time for you to tell me about how cool the new Metal Gear Solid game looks.
Rule 6: The GM does not have to be fair
Let me clarify that: the GM has to be fair in the overall scheme of things. No killing off characters without a chance, no unsolvable challenges (unless finding the way to solve them is the challenge), etc. But the GM does not have to be fair about the little things. So what if the rules say that the Jabberwocky can't attack more than three times in a round? If the GM wants to let it attack four times, because otherwise the party's going to waltz right over it, he can do that. And if he wants to say that the kingdom of Premblar is run by the orcs, even though the book clearly says its an elven village, that's his call, too. The GM is responsible for presenting you with the best story he can; assuming he's following the GM rules and making you the stars of the show, it's your responsibility to accept that story as its presented.
Hmm. Okay, this is long enough. As JeffD said about his rules, any comments, additions, or even arguments are welcome. Have at it!