The short story form is a nasty little critter. Because it involves fewer words, it's easy to fall into the trap of attempting one, then ending up with an overextended piece of flash fiction, which would have read better if it wasn't so drawn-out, or (worse) essentially a novel synopsis: too much plot crammed into a tiny space.
My writer's group just finished a challenge which involved writing a short horror fiction set in a hotel room, and it brought it home to me again just how hard crafting a short story can be. As we all know, writing is a breeze when the muse hits, but a professional ethic requires a writer to be able to write good work, all the time. Having to write a short (3000+) horror (not my usual genre) entirely located in a hotel room (I could cope with that one) was both a blessing and a curse - good, because restrictions help creativity flow, but bad because how in heck can you squash a horror story into a hotel room?
I was not the only one to encounter difficulties, and between us, the writers in my group got a few really good lessons in the art of the short story.
The first one was: don't try to squish a novel into a short story. One of my friends got very excited about writing in her favourite genre; so much so that she dreamt up a complicated back-story for all of her characters, which she deposed in flashbacks throughout the piece. The effect was a lot of what in TV is called 'potatoheading'; 'I know how you must feel, Myra, having lost your father at the age of two and spending your childhood in a series of foster homes, but you must understand that my desire for revenge - coming from my mysterious past when I was abducted at the age of ten and returned two years later, no-one knowing the reason why - overwhelms my sympathy.' (It wasn't quite that bad, as she really is a good writer. But you get the idea.)
My mistake was in doing the opposite - thinking up a back-story for my characters, then not including any hint of it. Having a deeply thought-out plot isn't much use when you get back comments like 'Who the heck is Alistair?' (sexily evil villain that saves the day in an is-he-or-isn't-he-evil twist) and 'Why is it important that she has a stomach-ache?' (a miscarriage).
From this, we learned that a short story shouldn't require foot-notes. A simple concept, really, but harder to learn than you'd think.
Another writer in the group crafted a marvelous supernatural piece about a man dying in a room notorious for its suicides. The piece was subtle and spooky without delving into full horror, but it did have one fault: the plot device of ghosts visiting the man to talk about the afterlife got a bit old after the fourth ghost. Introducing yet another round of 'Ghostly 20 Questions' took up valuable room in the piece which could have been devoted to atmosphere and description, and meant that although there were plenty of spooks in the story, there wasn't that much spooky.
Using an interesting plot device is one of the classic ways of giving a short story an extra kick, but relying on it too much can kick your audience right out the other side into boredom.
The other lesson learnt from our exercise came from the feedback, and it was the same lesson writers learn every time they get a wide range of feedback: people have different tastes. What constitutes an excellent short story for one person is what ruins it for another.
The main lesson learnt from our writing exercise came from completing the exercise itself. Don't be afraid of the short story form: corner it, hogtie it and sit on the little blighter until it behaves. Look up other writers' short stories, and see how they got them to behave. Find out what you like - stories whose writers keep them on a short leash, or ones that go out and misbehave? If you see a technique you like, do what writers have done for centuries and steal it. In the meantime, keep a close eye on your own creations and be prepared to discipline it appropriately: like all midget versions of creatures, the short ones are the most feisty.
Reference: Unfortunately - or fortunately, as the case may be - there is no formula for a short story. (Anyone who tells you that there is a formula for any creative exercise is a very scary individual indeed.) However, I have come across some very good advice from writer Edgar Allen Poe in his essay 'Philosophy of Composition', in which he outlines his process from desired effect to finished piece. Read the essay with a grain of salt, however - it is a little tongue-in-cheek.