Here's my list. Note that these really are my rules, or, more accurately, bits of advice that I give myself (and may or may not always follow). Some of them are pieces of advice I would cheerfully give anybody and everybody, but others are very much All About Me.
(I'm typing these in no particular order, but I may swap them around once I've got them all written out.)
1. "Write only what you would feel terrible not writing." Stolen/paraphrased from kerravonsen, back in the days of the Sentinels for Christ mailing list. I find that I have simply too many ideas (especially when it comes to fan fiction!) to be written in the time that I have. I doubt I'd be able to write them all even if I didn't have to work for a living. This is the rule I use to sort them into categories: the ones I need to work on, because I'll regret it if I don't; and the ones I just like, but could drop without feeling the loss.
2. Show, don't tell. I'm hardcore about this, personally. The point of writing a story is to let people experience what the character(s) is experiencing; to let them see through that person's eyes. Telling your readers what's going on hampers that, puts an unnecessary barrier between them and the story. There are exceptions, of course, for 1st-person narratives, for special formats (police report, scholarly excerpt, diary entry), and sometimes for characters who are very self-aware.
3. Infodumping is BAD. Give up as little as you can get away with at any given point in the story. This is similar to "show, don't tell," but in terms of information rather than action/emotion. If you tell me everything, I'm not going to be as engaged with the story as I would be if you let me do some of the mental legwork on my own. I may even feel that you as the author do not respect my intelligence. Also, in real life, knowing or not knowing things produces all sort of interesting problems--let that inform the drama of your story! Or the comedy of it. :-)
4. Details make the story more vivid, especially sensory details other than sight. I thank The Sentinel fandom and its fiction for really making me think hard about this, although I was already focusing tightly on senses other than sight in my descriptions. Details help in another way, too: you can use them to highlight what your character's mood or mindset is, by what he or she focuses on.
5. Kill your darlings. Perhaps this is why I never became a poet. I learned early on that elegance is no substitute for clarity. Anything that gets in the way of the story must go, regardless of how brilliant or beloved the phrase or line of dialogue may be.
6. Wishy-washy words weaken your writing. Wow, look at the awesome alliteration there. *g* By wishy-washy I mean words like "almost," "barely," "nearly," "kind of," or "nice." Qualifiers which pretend to be descriptive but really don't add anything to your phrase, and words which are so overused or bland as to be pointless even in context. The exception that proves the rule: when writing from a drugged/concussed/otherwise confused PoV. If the character can't find the right words, or his perception is off, that may be one way to indicate it.
7. Embrace structure, but don't let it rule you. I love patterns, adore finding ways to mirror concepts and dialogue in my stories, and in terms of structure I'm no different. But don't bend the story to fit some kind of preconceived pattern unless it will truly help you get your point across. Again, elegance is no substitute for clarity.
8. Don't be afraid to use your own experiences or to explore your own pain through your writing. Nobody will know how intimately it is yours unless you tell them; until then, it's just a story. You know your own story best, and may be able to tell that story with the most authenticity. And, bonus for the writer, there's a cleansing that comes with being able to walk through an experience and shape it yourself, rather than allowing it to shape you.
I may come back and add a couple of rules, but these 8 are the most important ones that I work from, at least right now.