There's a lot good advice on here so far, but the real question is, what level are you at now? If you're near the top of the game, the answer is different, if you're a complete noob.
That being said there are a few universal things that I believe are important. As someone else mentioned, bank roll management is very important. There are a number of players, good players that have gone bankrupt due to poor BRM.
Other than that, simple things really help a game. Play with a clear head; Pay attention to the action, even if you're not in it; and it never hurts to read one more poker book.