Got your PM and here we go.
First off, know the rules of the game. I am not saying to memorize the books. That would be silly but know the basic gist of of the game mechanics and know where to look when you do need to look something up.
Second, keep notes during the game. Make sure you know what is going on with the players. If you don't have good notes things can get a little chaotic, especially if you have a large group.
Third, before starting make sure you have all your house rules down. DMs like to tinker, but don't tinker midgame. One thing that kills a campaign is inconsistancy.
Fourth, if you do need to change a rule or three midgame consult the players first. make sure they are in the know and get their opinions. They just might like how things are, but be careful that the house rule isn't in their favor too much.
Oh, if you can bring a computer to the session or make use of one they are very nice tools to have. When I run a campaign I do so behind a computer screen instead of a GM screen. There are a lot of tools one can find online to aid in game mastering. Also if you need more resources you can't do better than http://www.enword.org if you are doing a d20 System campaign.