1. Don't keep your audience in mind while writing -- yes, even if you're writing for a ficafest-y type of thing where you're required to write something for a person. Sticking to a prompt -- whether a specific prompt or a list of kinks -- and writing to fit that prompt has nothing to do with the individual who provided the prompt, or any other individuals. Just write. Meet the prompt requirements, keep in mind the recipient's squicks, but don't think about anyone reading the damn thing. If you write something that's eyebrow-raising in a bad way, your beta will tell you, if she's good. Bottom line: if you work to impress an audience, it will show. OTOH, most people won't notice so if you're just out to get accolades or please masses of fangirls, ignore this entirely. I'm talking about writing, not crowd-pleasing.
2. Do consider criticism. If you don't, you're saying "there's nothing wrong with my writing". Honey child, something is always wrong with everyone's writing. If there were a perfect way to write, everything that can be written already would be, and no one would be interested in reading anything new. Criticism, especially from people who don't know you from Dumbledore, is one hell of a learning tool, if you know how to use it. That said, not all criticism is created equal. Sometimes people will say the stupidest things and fully believe that they're giving legitimate criticism -- notice that I didn't say you have to accept all criticism as true. But considering every piece of criticism, no matter how inane it sounds to you at first, can only help you. Unless you lack a spine and get so discouraged after the first bit of negative feedback that you vow to never write again. In that case, no offence, but you're probably better off not writing anyhow.
3. Don't force anything. If you have a clear, rigid outline and suddenly things aren't going according to plan, dump the old plan and draw up an improved one. If you are absolutely dying to use a witty bit of dialogue in a scene but there's no way you can fit it in without taking a major detour from your story -- save it for another fic. Pastede on is only fun on fandom_wank.
4. Do read a lot. And I don't mean fanfic, either, though fanfic by really good writers will do in a pinch. I believe the best way to learn anything is by example, and in writing, this is particularly true. I don't mean you're supposed to imitate what pro writers are doing. If you're still in the "imitation" stage of your learning to write, I hope you aren't actually sharing your writings with anyone because few things are worse than a rookie trying to imitate a pro. Your own style emerges only after practice, but you needn't share all the fruits of your practice with the world. Trust me on that one. One of these days I'll translate one of my early attempts at sci-fi, written at the ripe age of six. Then you'll deffo see what I mean. XD But yeah. No way around this one; reading a lot helps improve your writing -- not to mention it gives you an idea of what's original and what's cliche, etc.
5. Don't compare your writing to other people's. Someone else will always be better than you, and you do yourself no favours trying to pull yourself up by your bootstraps. Everyone has their limits; know your own. A good writer is not one who's better than everyone else. A good writer is one who does the best job she can using whatever talent and skill she's got.
6. Do remember that less is more. If you're explaining every single action, describing every single thing you mention, you're doing far too much. Stop and ask yourself -- what story do you want to tell? Then, cut everything that doesn't have anything to do with that story out. Then go over the bits you've cut out and select the ones that you feel add flavour, that special something that will make the bare-bones story yours. Put them back in. Fiddle. See if you can spot any themes or inadvertent symbolism -- crank it up, take it out, whatever works. Editing can be as much fun as writing, in this regard.
7. Don't tailor your writing to research. Some preliminary research can prevent having to rewrite huge chunks of text later, especially if you're writing about things you don't have a lot of first-hand knowledge in, but doing all your research before you sit down to write is for scientific literature, not fiction.
8. As a corollary to #4, do read reference materials (or at least have them at hand): dictionaries, books on grammar, idioms, etc. Books on writing itself are largely useless, because, to paraphrase Stephen King, most writers don't know what the hell it is we do most of the time, we just know it works. But objective reference materials can only enrich your language knowledge, and language is your primary tool whilst writing.
9. Stealing this one from snegurochka_lee. Don't post as soon as you're done writing. Leave it for at least a day or however long it takes for you to get it back from beta. Lose the "writing high" before you begin editing. The writing process is the heart and soul of writing. Editing is the mind of writing; best kept cool. If the extent of your editing is "reading it over once right after finishing, no beta", I mock you savagely (and probably don't read your fic). Some people self-edit extensively as they write; this is marginally better, but not by much. Self-editing while you're writing causes bad sentence structure, possible continuity errors and occasionally jerky narrative that discerning readers will notice. Do you have to have a beta? Not absolutely, no, if you're strict about editing and are your own worst critic. But there's a very good reason that major publishing houses employ editors.
10. Do have fun. Why are you writing, if you're not enjoying the process? Even if you're tearing your hair out in frustration at a particular passage, even if your characters have all set up hammocks in the back of your brain and are swinging about idly and refusing to cooperate (*glares at Jens and Carlos, wannabe heroes but really major SLACKERS >:0!*)... even if you're looking at a page full of text and going "wow, do I suck or what?" Writing is frustrating, it drains energy, it fucks with your self-esteem and makes you feel miserable when your story turns a corner you didn't expect and you end up writing about stuff that bothers you. To name just a few things that make writing one bitch of a hobby. But there's always a thrill, a spark of something -- something different depending on whom you ask, but it makes writing so enjoyable and fulfilling, at the end of the day.
*goes back to writing*