Just a quick tip today - when writing your screenplay, it might help to keep in mind the rating of the movie when working on your dialogue. Certain words will automatically get you a certain rating.
For instance: you can say "hell," "damn," "ass" and "shit," in a PG film, but "Son of a Bitch" will usually get you PG-13.
In PG-13, you can say every swear word but "fuck." You're allowed one instance of "fuck," usually, but as long as it's not in a sexual context. In other words, a character can say, "Fuck you!" in anger, but he can't leer at a girl and say, "I want to fuck you." I have to admit, I'm not sure what the rules are on "the C word," but that seems to earn a film an R as well.
Graphic sex will earn you an R faster than graphic violence, so be especially careful when writing scenes that make heavy use of anal and/or oral sex. I see these a lot in script that seem like they'd be aimed at a teen audience - which means that PG-13 would be the desired rating. Yes, occasionally there'll be a successful R-rated teen sex comedy, and the success of The Hangover earlier this year (to say nothing of Judd Apatow's oeuvre) means that R-rated comedies in general might be on their way back in - but for a long time, studios were aiming for that PG-13 sweet spot. If the primary audience for your film is teenagers, you should aim there too.
So don't over use "fuck," and a word to the wise - anal sex scenes are never as edgy and clever as most writers seem to think they are. There was one week where it seemed like every other script I read had a particularly gratuitous example of such a scene. Without them, the scripts would have been soft PG-13, easily. Because of that, not only did the scene feel like a writer's indulgence, but it stuck out like a sore thumb.
So know the audience you're writing for, and don't step over certain lines just for the sake of doing so. Yes, language can always be cut back, but if the word "fuck" appears more often in your script than the word "the," it might be time to do some trimming.