The Five Commandments of GMing

Like I said before, GMing is an art. Art has rules, and so does GMing. By rules I mean things that help for overall enjoyment of the game. They allow the game to be everything it can be; that is the point of rules, after all: they enable the greatest experience. These rules hold up no matter the situation. These are not preferences: if you're not doing these things then there is something legitimately wrong. So follow them!

1. If everyone's not having fun, no one is having fun. This means you too! This isn't a corny reminder that this is a game. I'm being serious. GMing can be a stressful hobby. You've got all sorts of things to consider, and they change from game to game. Different people want different things in different campaigns and games. Rules need to be followed and learned. And then there's the book-keeping. Oh my good sweet Lord, the book-keeping. So for the love of all that is good and holy remember to lighten up and have fun! Not only will it help you save your sanity, it is the point of GMing. For all it's work GMing is an awesome hobby, and I wouldn't trade it for any other hobby.
2. You are not the players' enemy: you're their fan! Your job as the GM is not to make life hellish for the players, ever. Some GMs talk like they're the a player's worst nightmare. And some really are. Do not be that guy. It's a bad place to be, to be your player's worst nightmare. That means that, in general, you're an asshole that they really don't wanna play with. Allow your players and you to have fun, because if one side isn't having fun no one is having fun.
3. It's a different story for the player's character's, however. And this is where you get to be devious. The characters are silly putty in y'all's hands: make use of it! The characters are fictional constructs in a story: stories are good when characters suffer. So be a jerk to the characters all you like, but never at the expense of the players. Remember, if not everyone is having fun, no one is having fun.
4. Embrace your game's strengths. This is vitally important. Take what your game can do and embrace it. Does it have a good exploration system, but no arguing mechanics? Then feature lots of exploring and no arguing. Good fighting system but no exploring? Lots of fighting then. Make sure that your players are aware of this: if someone wants an intrigue heavy game, 4th Edition Dungeons and Dragons really isn't going to work for them. Talk about this with your players, see what they want.
5. Table rules are exactly that: rules. Good manners are key, and they must be defined by you, the GM. Basic rules are universal: no talking over other players, if you have a problem wait until after the session, and if you're going to miss a session tell the frickin' GM! Now, you'd think that these rules are self-evident. I am here as the bringer of bad news: not only are these things not self-evident they must be enforced as if you're dealing with a bunch of cats. Obviously your mileage will vary based upon the people you're dealing with, but even so: do not assume anything when it comes to manners.

There are plenty more things that can be said, but they're more like preferences, things that work at some tables and games more or less than others. The above rules really are just that: rules. Follow them at all times and your games will go well.

EDIT: One of my commenters on Facebook linked me this picture. I have now declared a new rule!

Rule 6: If your players keep being ignorant jerks, throw the following at them...