On a /fmg/ (Filmmaker General?) about a week ago, I mentioned that I was given the opportunity to write a script treatment for a producer to make a low-budget film. I turned it in, guy LOVED it, and it's being produced soon. I had to sign a NDA and I can't be involved in production/post because of union bullshit, but I got paid and I have been told I can distribute copies, clips, stills, and whatever once they've finished. Weird rules, but I dig it.
Some tips I've learned from this experience:
(1) There are no rules. There are general guidelines, parameters, and templates you should follow, but the specific, ridiculous bullshit that I learned in film school? Nope. Not even a thing. You write to please the person who can give you a job. Not the person who taught you in college.
(2) ACTIVE VOICE for treatments, doesn't matter for screenplays. "He opens a laptop" is different from "He is opening a laptop." Always present tense, though. Never past tense (except in dialogue).
(3) This is not a novel. This is is a screenplay. Using pretty prose language is never necessary. In a novel, sure "The rapturous, spellbinding landscape of the billowing ocean waves cascaded against the whatever-whatever-whatever" is appropriate. In a screenplay, "The oceans waves crash." is more than enough.
(4) Finally, outline everything before your first "FADE IN." It'll keep you organized and on track.
That's all I learned from this process of "professional" screenwriting. Before, I was only writing for either productions I, myself, would be creating or friends. This was a completely different ballgame.
Also, word of advice - Celtx is free and awesome.
Good luck, and happy writing.