If you see me breaking one, call me on it after the session. So that I can properly smack myself.
1) Never change a player's success because it is inconvenient for them to succeed. Also, tying into this, always have your NPCs statted out from the beginning (at least class level) so that you aren't tempted into raising their level later when the players unexpectedly attack them.
2) Know the characters that the players are trying to be and make their abilities useful (especially for social/ charisma-based abilities: it's easy to write off Diplomacy checks, even if successful).
3) Know the rules for your player's abilities (Ride is a bitch).
4) Have a plan for each session, but be ready to chuck it if the players are trying to do something else (avoid rail-roading at all costs). Hopefully the players will not go out of their way to screw up your plans, but they may choose to do things in a different way. Be ready to change.
5) If your monsters use loot, leave it for the players upon defeat.
6) If your NPCs use a certain variation on rules, your players have to be able to as well.
7) If someone is feeling left out, make an "encounter" where they can shine. (Even combat-centered characters might want to feel useful in the story.)
8) Don't pick on your players all the time. Sometimes they just want to feel like their characters are awesome.
9) The majority of people in the world are not high-level. (Remember this for guards, brigands, etc - usually they'll be low-level NPC classes). This also plays into making your player's characters feel awesome: once they get to a high enough level, they can scoff at your average bandit and your average guard. Don't pull an Oblivion and level up all the encounters as your players level. This is all especially true if they started at low levels and once had to worry about the lower level things.
10) Make the game for the players, not the other way around. So, listen to critiques and admit to mistakes. Hide things about the game, not about how you run the game.
1) If you have issue with something the DM has done, bring it up after the session.
2) Trust the DM, because really you don't have much choice when it comes down to it. If you think you can't trust them, see rule 1.
3) Remember you don't have to kill everything.
4) Sleeping in dungeons can be dangerous - find a good place to do it, and be ready for possible repercussions.
5) If you're going to play Evil, do it the right way: insidiously.
6) The more you tell the DM about your character, the more they can tie it into the story (I still remember Dhymil, my first D&D character, finding out the Order of the Crimson Flame that killed his family was going to be a major story point). Talk to your DM if you want a certain magic item/ life-changing event/ PrC/ something later in the character's life. Usually they'll
try to be accomodating.
7) Roleplay with everyone in the group - don't make a character too isolated/ secretive: RPing someone who's always quiet isn't as cool as it seems in concept (Dhymil was initially the quiet-cool in my first campaign: it lasted 2 sessions).
8) Combat isn't just about winning - it's actually a lot more fun if you can RP it out the way your character would. Don't turn off the RP just because you now have to kill something. The best thing to do mechanically isn't necessarily what your character would do. The DM should do the same thing, and set the standard.
9) Know your character's abilities. That way you can always be ready to use them, and it'll speed up game-play.
10) Don't intentionally screw with the DM. The poor guy has a lot to work on *grin*. (However, do occasionally ask him for the name of a random NPC/town/country. DM's love it when you do that.)
~DD