Joss Whedon, writer of Buffy the Vampire Slayer, Firefly, the upcoming Dollhouse, and notoriously brilliant rewriter of just about any major screenplay you enjoyed (I'm looking at YOU, Iron Man) has written his Top Ten Tips for Screenwriting.
Pay attention. Even if you're writing a book instead of movie, most of this applies to you. Pay particular attention to Tip #1. It's a doozy.