This article from MovieBytes (a great resource for information about screenwriting contests) has some solid tips about how to improve your screenwriting, but this tip is the one that stood out to me most:
Don’t Be the Perfectionist
Examine your process – how you write the script. Let’s say you’ve outlined your script. You’ve blocked out time and are coming at it with good energy. You barricade yourself in with a copy of Baudelaire’s Flowers of Evil, 18 bottles of Pabst Blue Ribbon or whatever it takes to get you rolling. The pages come out, but look like crap. What the $#@*! Why?! Remember this scene in Amadeus?
God speaking to you lately? It doesn’t happen. There are going to be so many rewrites, polishes, trims, tucks, cuts…the script in constant revision mode. Don’t be a perfectionist. Don’t keep rewriting the same 30 pages.
I’ve seen good writers lose confidence this way. They can’t get the scene down, but they won’t let it go. You have to push forward. That’s the purpose of the rough “discovery” draft. Push forward, say everything you want to say in rough form. If, at the end, you’re looking at 140 pages, so what? You’ll know what needs to be done by the time you reach the end. Don’t censor yourself. Push out. Get the rough draft done, then refine.