I've been writing screenplays for 7 years and I've sold 2. Nothing ever happened with them, however. Anyway.
1. GET YOURSELF FORMATTING SOFTWARE!!! That's the most important thing. Producers won't read your screenplay unless it's in an EXACT format. Try Final Draft or Sophocles. You can download trial versions off of DOWNLOAD.COM.
2. Keep it at 90 - 110 pages. Everybody says 120. They'll prefer a 90 pager to a 120 pager anyday.
3. It HAS to be structured. They want 3 acts. Any screenwriting book will teach you this structure. STICK TO IT! Producers have people that read these things for them and they want to see format, page counts, and structure breakdowns.
4. Join the Screen Writer's Guild. This will give you protection and immediate access to thousands of agents and producers.
5. DO NOT BASE YOUR SCREENPLAY OFF OF ANY BOOK, PLAY, COMIC BOOK, ETC.. WITHOUT GETTING PERMISSION FIRST!!!
6. Be prepared to write many, many query letters. Make sure you can spell out what your movie is about, what the overall arc of those characters will be, and who your target audience is in 1 or 2 paragraphs.
7. Have a GREAT TAGLINE for your movie. When they talk to you they want you to sell them your MOVIE, not your screenplay. They want you to excite them.
8. Be prepared for a lot of rejection letters.
9. Don't settle for anything less then you deserve. If you think you have a great screenplay, then don't sell until you get what you want.
10. Be open and work with their criticisms, even if they suck. Your screenplay will get re-written many times before it's shot. That's the truth. And if you show that you're willing to work with them and can show excitement for their ideas and creative imput then you stand a chance at being the guy who gets to do the re-writing.

GOOD LUCK!!!