Billy Wilder would be on every short list for greatest screenwriters ever. I always gotta root for the writers who get to direct, and he was a hell of a director, too. Wilder managed to make classics in several genres, and would get bored doing the same thing for too long. He said when he was in a good mood, he'd write a tragedy, and if he was sad, he'd write a comedy. Even some of his lesser-known scripts have extremely original or clever touches (Ball of Fire comes to mind), and he had a knack for story developments and character moments that were both unexpected and satisfying (as in Stalag 17 and Some Like It Hot, but most of all The Apartment).
In any case, from Conversations with Wilder (via Lists of Note), here's his advice for screenwriters (and writers in general).
1. The audience is fickle.
2. Grab 'em by the throat and never let 'em go.
3. Develop a clean line of action for your leading character.
4. Know where you're going.
5. The more subtle and elegant you are in hiding your plot points, the better you are as a writer.
6. If you have a problem with the third act, the real problem is in the first act.
7. A tip from Lubitsch: Let the audience add up two plus two. They'll love you forever.
8. In doing voice-overs, be careful not to describe what the audience already sees. Add to what they're seeing.
9. The event that occurs at the second act curtain triggers the end of the movie.
10. The third act must build, build, build in tempo and action until the last event, and then – that's it. Don’t hang around.
Reportedly, he also delivered one of my favorite aphorisms: "If you're going to tell people the truth, make them laugh, or they'll kill you." (I've seen it phrased a few different ways, and also attributed to Shaw, Wilde, and others, but it certainly makes sense from Wilder.) I've always been fascinated by where comedy and tragedy intersect or change places, and Wilder understood that much better than most. (I still need to see some of his films, but considering how prolific he was, that's not surprising.)