I just wrote a long response to this and I don't think it went through! Which brings me to my first piece of advice, always SAVE. I gave some damn good advice too.

Basically what I said was that it's most important to FINISH your script. I talk to so many aspiring writers that have an idea, or have an outline or 20 pages of a script. It's so important to just finish so you can see it as a whole and then go back and revise.

And never be too precious because the script will get noted up by producers, studio, directors, actors! It is an ever evolving thing! It's going to change from your original draft. Or maybe I'm just a terrible screenwriter, jury is still out on that one.