1. Start as close to the end of the story as possible.
2. The first sentence should aim the story.
3. A short story is about the most important event in someone's life. A novel is about the most important period in their life.
4. It may be in your head, but if it's not on the page, it didn't happen.
5. Reduce the plot to a single sentence. Then you'll know what's important and what isn't.
6. Don't have too many characters whose names all start with the same letter.
7. Don't be afraid to let yourself write crap. (at first, anyway!)
8. And my favorite, which was attributed to Howard Waldrop: You've got to find the Thing. (this one is worth some serious pondering time! the "thing" seems to be like the theme, but not entirely.)
Oh, and some of the first advice I ever received from a professional editor:
Three Rules of Short Stories
1. Why do we care? (about the character/s)
2. What's the failure cost? (of their actions or inactions)
3. What do we win? (hopefully, character change)