Did you know there are rules for screenwriting? Actual rules. And when you don't follow the rules, you get beaten with a yardstick -- or ignored by Hollywood. Whichever hurts worse. Whenever I fancy myself a screenwriter, I hire an assistant to help me follow the rules. OK, "hire" is perhaps too strong a word; "open" may be more accurate.
Celtx is a terrific little piece of software, absolutely free, that will write scripts, storyboard scenes and sequences, sketch setups, develop characters, breakdown & tag elements, schedule productions, and prepare and circulate informative reports for cast and crew. (Yeah, I plagiarized that description from Celtx' website. Ssshhhh.) In between screenplays, it also writes comic books. Not kidding.
Now if Charlie Sheen could just download an assistant to help him follow the rules, all would be well in Hollywood.