Keep your campaign world consistent with itself. Player immersion in the campaign is a big part of the fun when it comes to playing D&D.
Seconded: internal consistency is vital! This includes houserules or on-the-fly rulings: think twice, rule once; as once it's done it's part of the game and to change it later (usually) violates internal consistency.
I might be echoing some things said earlier, but for me:
1. Be ready at any and all times to hit the curveballs thrown at you by the players. Just because you've spent all week prepping The Most Excellent Dungeon Ever(TM) doesn't mean for a second that your players will go anywhere near it; they might instead decide on a whim to go explore the south coast. And you've got to be ready.
2. Make it clear up front that bad things can (and probably will) happen to the PCs either individually or collectively as the campaign goes along; to forestall any bad feelings when those bad things inevitably do happen. (side effect: when bad things happen the good things that happen just seem that much better)
3. As long as it's in character, let 'em play. By that I mean let 'em fight each other if they want to, let 'em make love to each other if they want to, don't force them to be noble heroes if they want to be bloodthirsty marauders, and if they want to spend tonight's session arguing amongst themselves instead of getting on with the adventure, let 'em (and enjoy the break!).
3a. By the same token, however, if for whatever reason they're the sort of players who need to be led by the nose from one adventure or scenario to the next then by all means fire up the locomotive and get 'em on the rails.
3b. For all this, try if possible to have players who know each other outside the game (or who at least you know), so you have half an idea what to expect.
3c. If you're in the lucky position of being able to choose your players (as in, more people want in than you have space for) always take the entertainers over the serious ones; more fun for all.
4. Make sure your game world lives and breathes when the PCs aren't around as well as when they are. Achieving this can be as simple as having some minor background things change over time...the tavern where the adventurers first met has been closed by the city watch while the PCs were off adventuring, for example...whether these changes directly/immediately affect the PCs or not. If the party has a particular town they tend to base out of, maybe each time they return have a brief local news report ready...if the players are interested...to let them know what's been happening while they were away (and once the campaign gets nicely going the results of previous actions by the party can be embedded in these reports).
5. No special snowflakes. This includes both within the party (don't favour any one character) and within the world - just because the PCs are PCs doesn't (or usually shouldn't) make them any different from the rest of the game world...they're part of it, rather than superior to it, if that makes any sense. (exception: if your storyline specifically revolves around one or more PCs being special, go for it; with the caveat to be ready to adapt quickly as inevitably those are the PCs that will find a way to die first)
Lan-"and above all, you're never good enough that you can't be better"-efan