Short fiction is the "garage band" of science fiction, claims Tor Books editor Patrick Nielsen Hayden, so it's time to step on that fuzzbox and thrash as hard as you can without knocking over your mom's weed-trimmer. Actually, I think Nielsen Hayden was referring to the fact that you can try more crazy experiments in short SF than in novels, because of the shorter time commitment of both writer and reader. But how can you become a super-master of the challenging form of short fiction? Here are a few suggestions.
I wouldn't claim to be an expert on short fiction writing, but I have written over a hundred of the little fuckers, a large proportion of which have been science fiction-y. Here are a bunch of do's and don'ts, that I discovered the hardest way possible.
World-building should be quick and merciless. In a novel, you can spend ten pages explaining how the 29th Galactic Congress established a Peacekeeping Force to regulate the use of interstitial jumpgates, and this Peacekeeping Force evolved over the course of a century to include A.I.s in its command structure, etc. etc. In a short story, you really need to hang your scenery as fast as possible. My friend and mentor d.g.k. goldberg always cited the Heinlein line: "The door dilated," which tells you a lot about the surroundings in three words. Little oblique references to stuff your characters take for granted can go a long way.
Make us believe there's a world beyond your characters' surroundings. Even though you can't spend tons of time on world-building, you have to include enough little touches to make us believe there's stuff we're not seeing. It's like the difference between the fake house-fronts in a cowboy movie and actual houses. We should glimpse little bits of your universe, that don't necessarily relate to your characters' obsessions.
Fuck your characters up. A little. Just like with worldbuilding, you can't necessarily devote pages to your characters' childhoods and what kind of underwear they wear under their boiler suits. Unless your story is really a character study with a bit of a science fiction plot. I used to have a worksheet that included spaces to fill in in info about each character's favorite music, hatiest color, etc. etc. Never filled those out. If I'd tried to force myself to come up with a favorite color for every character, I would have given up writing. But do try to spend a bit of time giving all of your characters some baggage, just enough to make them interesting. Most science fiction readers are interested in characters who solve problems and think positively, but that doesn't mean they can't have some damage.
Dive right in — but don't sign-post your plot in big letters. When I started writing stories, my early efforts meandered around for pages before something happened to one of the characters to make him/her freak out. And then the rest of the story would be the character(s) dealing with that problem. And then, as I got more practiced, I found the foolproof map to awesome storytelling: introduce whatever it was that was freaking out my characters in the very first sentence of the story! And then the story could be about them dealing with that problem, until they solved it in the very end. It was so perfect, how could it fail? It took me another year or two to realize that plunging the characters into the story's main conflict right away was just as boring, in its own way, as the ten pages of wandering in circles. The best short stories I've read are ones which start in the thick of things, but still keep you guessing and let you get to know the characters before you fully comprehend the trouble they're in.
Experiment with form. Short fiction isn't one form, it's a whole bunch of forms jammed together according to their length. Short stories include your standard 3,000 word mini-odyssey thru the psyche. But they also include flash fiction (sometimes defined as under 100 words, sometimes under 500 or even under 1,000.) And those wacky list things that McSweeney's runs sometimes. In fact, for a while there, postmodern short fiction was all about the list, or the footnotes, or the krazy monologue, or the story told in office memos. Try writing super-short stories of only 10 words, or mutant essay-stories written by a fictional person. Also, if you always write third person, try first person. Or if you're always doing first person, try third.
Think beyond genre. Often the best genre fiction is the stuff that cross-germinates. Pretend you're actually writing your story for the New Yorker, and try to channel George Saunders or even Alice Munro. See how far you can go towards writing a pure lit piece while still including some elements of speculation. Or try writing your story as a romance. Or a mystery. Imagine it as a Sundancey indy movie.
Don't confuse your gimmick with your plot. You may have a great idea for a piece of future technology, or some amazing mutation that turns a whole bunch of people into musicvores who survive by eating your memories of rock concerts. Maybe you have the most original basic premise evar — but that's not your plot. Your plot is how your new widget changes the people in your story, and how it affects their lives. Or what decisions your people make as a result of this new technological breakthrough.
Don't fall into the character-based/plot-based dichotomy. People, especially in writing groups and workshops, will try to categorize stories as based on either plot or character. This is a poisonous idea that will turn you into a cannibalistic freak wearing a belt made out of human spinal cords. There's no such thing as a character-based story or a plot-based story, because every story has both. Even the most incident-free Ploughshares romp or the most twisty thumpy space opera tale. If you start thinking that stories can be categorized into either pile, you'll end up writing either eventless character studies or plot-hammer symphonies starring one-dimensional nothings.