1. No 2-dimensional, cartoon villains.  (Think the Emperor from Star Wars.)  These aren’t real people.  People have motivations.  People are complicated.  Villains should be people.
2. All narrators are unreliable.  Even without dishonesty, events will look different from different perspectives.
3. Don’t be afraid of the supernatural.  Use it to tell stories.
4. Don’t overuse it, either.  It will get stale and lose its capacity to inspire wonder and instill fear.
5. The world doesn’t make sense.  Not everything gets explained or tied up in a neat little package.
6. Alienation.  People don’t completely understand one another.
7. Listen to your characters.  These are their stories.
8. There is no such thing as fate.  The good guys can lose.
9. Really examine your characters’ plans; look for loopholes and flaws.  Either plug them or explain why they don’t matter.
10. In the service of the story, all of these rules can be broken.

* These rules are merely descriptive, not prescriptive.  Other people should follow their own style.