The game is not yours, it's everyone's.
Collaborate, work together, have fun. As a player, I probably didn't show up to listen to what you had planned, I came to play. As a GM, I really don't want to be entirely responsible for anyone's fun. The great thing about games is that they provide a framework for us all the contribute to a great experience in different ways, but none of those ways is more or less valuable.
If I could sneak in a close second, it would be:
GM the game you're playing
Which sounds like common sense, but it's actually one of the best ways I've improved my GMing. You don't run 4E like you do 3E, or GURPS like you do Burning Wheel, or whatever. Keep in mind the game you're playing, and run that game.