1. Keep writing.
2. Don't stop writing.
3. See # 1 and 2 above.
4. Do not stop to edit. Editing is a separate undertaking. Stop worrying. In other words, see # 1, 2, and 3 above.
5. Learn the rules. I'm talking spelling, punctuation, and grammar here. If you don't know the rules, you can't break them intelligently when the story calls for it. There's a big difference between breaking the rules for a reason and breaking the rules because you've been too @#$%& lazy to learn them.
6. Don't let anyone tell you what to write or not write about. It's your writing.
7. Let the humor come naturally. Don't force it. If you force it, it will be lame. There is no way around this.
8. Let the muse speak through you. You are a vessel for the creative forces of the universe. Don't edit the @#$%& muse. In other words, see # 1, 2, 3, and 4 above.
9. Contrary to what others have said here, there is nothing wrong with fan fiction. It can be done well. It isn't often done well, but it can be done well. And playing in someone else's sandbox can be a lot of fun. Go ahead. Prove 'em wrong. Write a really good fanfic. Just deal with the fact that you can never publish it for money.
10. Don't be verbose. Don't use more words when less will do. I mean it. Seriously. For reals.
Bonus: 11. Have fun, for cryin' out loud!