Here's a few things that I've learned during my time in writing fanfiction that you may or may not find helpful.

>>Get a beta reader.  A beta reader, contrary to popular belief, is not a glorified spellchecker, although they can serve that purpose.  A beta reader is someone who knows your fic, knows what you are capable of, and knows what good fic should be.  A good beta reader should always be able to push your fic to its next level.  Sure, you could write a fic without a beta, but why would you want to?

>>Proofread, proofread, proofread.  It's been my experience that a good beta spends so much time on your fic itself that she might miss grammatical and syntax errors.  Even if she's looking out for that stuff, everybody's human.  Don't give into the adrenaline rush, must-post-now feeling you get right after finishing a fic.  Walk away from it for at least a few hours, come back, and go over it with a fine-tooth comb.  You'll catch a lot of little mistakes that way.

>>Watch the show.  Pay attention.  How much is readily apparent, and how much is subtext?  What are the characters saying, and, more importantly, what aren't they saying and why?  What about the character you're writing?  Watch all his/her scenes.  Watch them again.  What do they eat for breakfast?  What kind of music do they listen to?  How do they live, hate, love, behave?  Ask yourself random questions- how would the character respond to this movie, this restaurant, this political dispute or religious question?  If you don't know, rewind your tapes and watch again.

>>Tread carefully in non-canon issues.  Most of us prefer unconventional relationships and alternate universes- that's the joy of fanfiction.  But fanfiction is not original fic, and you still have to work within the parameters of certain expectations. Characterization is everything. You want to make your character behave in a way that they never would on the show?  Go for it.  But remain true to the character, and justify their actions in terms of what you know about them on the show.

>>Read fic.  Especially good fic.  Join mailing lists.  Search through fanfiction archives.  Ask people what they recommend and where to find good fic.  (If you ask me, I'll say Eterniata, because it covers such a wide range of characters and pairings, yet retains its standard of quality across the board.)

>>Don't be afraid of fanfiction authors- even your faves.  We all get starstruck sometimes, but fanfiction authors are just normal people, too.  If there's someone whose work you admire, let them know.  Send them feedback.  Ask them questions.  Pick their brains.  Chances are good they'll be very accomodating.

>>Challenge yourself. Write a character or pairing that you aren't interested in or can't stand.  Don't worry if the resulting fic is crap; you've still learned something from the experience.  Join a challenge or improv list.  Set deadlines and goals for yourself.  You might not meet them; that's okay.  It'll strengthen you as a writer.

>>Give it your all.  If it doesn't feel finished, it probably isn't.  If you feel you're going halfway, go farther.  If you think you have more to say, say it.  There's a lot of fanfiction online- a lot- and we all know that a big percentage of it's crap.  Work hard to make your fic the best it can possibly be.  It's worth it.

>>Don't get discouraged. Don't worry if you go through writer's block- it'll pass.  Don't worry if you think you've run out of ideas- you haven't.  Because fanfiction is something most of us do as a hobby, it's easy to sink into dry spells, especially when real life gets in the way.  These spells don't last.  Don't give up on writing just because you're not getting "enough" feedback (there's never enough, trust me) or because someone flamed you.  It won't help you be a better writer, and everyone will think you're immature.  If you're only writing for praise, then something's wrong.  Fanfiction is a community process, true- but write because you love it.

>>recommended links: Blurred Vision has some great essays on writing fanfic.