Do you read a lot of short stories? You might find it useful to pick up or borrow a copy of the "2007 Best Short Stories" Anthologies or something, or the Pushcart Prize book for this year, etc.
There's always the "how do you go back to short" thing, too. I've always written shorter stuff, but since I've been working on much longer WIP for just a few months, even if I think I'll give it a rest for a few days & write a short story, I can't do it. I start making the characters too complicated, the story too long, and then I'm putting it in my vignettes folder to come back to later as another novel idea.
A short story should capture a single moment, an emotion, something real. A small event that signifies something bigger. I just read a lovely one in All-Story called "The Stars are Bright in Texas" where a lady is talking about house-hunting in Houston and going into the oil business. They go house hunting & along the way she is talking about having just miscarried her first baby - she's still taking antibiotics. They find the PERFECT house, but someone outbids them. They are on their way back home & telling each other, "there will be another one." Then she says, "But we wanted the one that was gone." And you realize she means more than just the house. It made me cry when I read it. It's ostensibly about house-hunting in Houston (and I bought the issue because lately I get so homesick for Texas I'll even buy a magazine with a short story about somewhere in Texas) - but really it is about loss.
A really good novel transports you for a long time into another world & tells you a great story. A really good short story will capture a single scene that says something about life in general.
Not everyone would probably agree with me, but that is my opinion. Uncle Jim says that a short story is a piece of wood, and a novel is a box. If you build your box and it doesn't square up, you can take away pieces, put others on, rearrange, whittle away, and make it into a good box. But with a piece of wood -- it's either a good one or not, and if it isn't good, you just have to start over. I didn't agree with this assessment at first, but now I can kind of see his point (oh, go find his commentary on it; it's better than my paraphrase).
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