I was looking for the quote about letting the audience add up two plus two themselves (it turns out not to be Wilder, but Ernst Lubitsch), and I found this fantastic list.  Although it's written for screenwriters, if there's anything on it that doesn't apply to writing novels, I'll eat my hat.

Yes, the famous Hat of Fate. I'll actually eat it.

Billy Wilder’s Tips for Writers

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. The event that occurs at the second-act curtain triggers the end of the movie.

9. The third act must build, build, build in tempo and action until the last event, and then –

10. — that’s it. Don’t hang around.

Taken from “Conversations with Wilder“, by Cameron Crowe