Allow yourself to write a bad first draft. What’s important is that you write it down. No one other than you should see the first draft so you can be as bad as you’d like. Try following that strange idea you had last night while showering for a 10 pages to see where it goes or create the waiter who reminds you of weird Uncle Ralph just a little too much. Write freely knowing that a lot of your first draft (if not all of it) will be rewritten before a second set of eyes see it.
I think I’ve said this before, but read your scenes out loud. That’s where I find most of my mistakes. If it doesn’t read aloud well then you need to either correct it or rewrite it. While reading your first draft you should notice the quality of the writing and storytelling improving as you flex your creative muscles. You may find yourself keeping more of the second half of your screenplay because it’s actually better than the first half of the screenplay.
Finally remember to keep a copy of every version of your screenplay. Buy yourself an external hard drive just for your writing, but that’s another scriptip for another day.