So, about rules: if you are even bothering to read this, chances are that you are past the point where following mechanical rules is going to do you any good. There IS such a point, don't get me wrong, but it is pretty early on in the process of development as a writer. Rules are helpful for someone who is making a lot of the common clunky mistakes that newbies have made from time immemorial, because even just following a bunch of rules blindly will scrape a lot of that ugly barnacle stuff off your stories, hopefully improve your ear, and make it easier for you to get betas and feedback. But that is fairly easily done, and then you've got whatever gains you're going to get from them.
On the other hand, if you are bothering to read this, chances also are that you would still like some kind of concrete advice to follow, rather than just finding what works for you or having fun -- both of which are absolutely the right advice, just not concrete. So, with the caveat that I am only qualified to dispense this advice so far as you (a) like the end product, which is my own work; and (b) believe that I actually understand the process of my own development enough to pick out the important bits of it, which is by no means guaranteed, here you go:
- Play around.
- Beta passionately.
- Write and post a lot.
The other nice thing about deliberate experimentation is, if you tell people in your story post "hey I am trying X in this story, tell me how X worked for you" -- you are MUCH more likely to get valuable concrit from people on that specific point, both because it tells people you really mean when you ask, and because it gives them something specific to look at.
Passionate beta: Beta for people whose stuff you already really like. This is key. Do not beta for someone if you don't actively enjoy reading their stuff. Don't look so much for "mistakes" (eg typos, plot holes, canon errors, wandering POV, unrealistic physical actions, passive voice, show-not-tell, etc). Instead, look for things that are not yet GREAT. Things that you might happily speedbump as an end reader, but that aren't working as well as they could. Sections you skimmed because they were kind of boring, dialogue that vaguely felt wrong or a line that fell flat to your ear, action scenes that weren't exciting enough or porn that wasn't really hot, a plot twist that didn't quite hit you as hard as it should have, a vague feeling at the end that the story could have been better, etc. Try and figure out why those bits aren't working quite so well. Try and come up with concrete suggestions to fix them, and reasons for those suggestions, but if you can't, at least point them out to the author, and try and figure it out with her.
(Note: for this to go at all well, you have to find someone who WANTS passionate beta. Otherwise it is like trying to force-feed someone chocolate cake.)
Write and post a lot: Practice always helps, obviously, and part of that is getting in the habit of finishing things and kicking them out to fend for themselves. cesperanza finishes literally everything she starts. I don't, myself, but I do regularly push through and find the endings of stories, and that has helped me tremendously. Also, I think implied in "a lot" is that every story does not have to be better than the last. Every story CAN'T be better than the last. They're planes on a runway. Some of them are 747s and some of them are commuter jets and some of them are six-seater Cessnas with mechanical problems, and every once in a while you get the Concorde. But the Concorde can't take off until you've got the five commuter jets and the Cessna out of the way.
I'll also throw in one more general piece of advice: try other kinds of creative work. Make icons or wallpapers, draw or paint, learn how to vid. (Vidding has absolutely made me a better writer -- it has made me think about color and movement and pacing in a completely different medium, and has made me better able to think visually, about the physical composition of scenes. So did designing a computer game and working on software architecture.) Make livejournal layouts or websites. Go research and write wikipedia entries. Play an instrument, make mashups. Learn how to cook or study a new language. Take photos. Write poetry. Dance. \o/
And don't take this advice or any other writing advice if you don't find it fun. I can freely prescribe this particular set, because this is how I get my fun. The feedback for a story afterwards is the fabulous dessert; the writing and betaing is my delicious philly cheesesteak sandwich. *g* Not that a rough beta can't be painful -- but it should be painful like a really thorough workout with a trainer who isn't letting you off easy, where you bitch and moan through the last few sets, but then once the aching has subsided you feel all powerful and smugly proud of yourself. Delayed gratification is fine, but if it's just a tedious chore with no real payoff for you anywhere along the way, then forget about it.
Now I go take my own advice about finishing things. *cough*