Uberslacker
The other thing you need to keep in mind is to keep your short stories to one story only. Novels are braided short stories that come together to make something bigger. "The Yellow Wallpaper" is a story merely about an oppressed woman's descent into insanity and her yellow wallpaper's relationship in that descent. Nothing else is told. Nothing else is relevant, even though surely there is more to this woman than her obsession with yellow wallpaper. But the author is focused; she only had one story to tell.
I'd check out that book Kathleen suggested, too. I haven't read it, but it's on my list of recommended books that I should read when I get a chance.
Take an "episode" from your "epic". Examine it, see how to make it "stand on its own." Voila! a short story!
Remember, the initial "episode" of any narrative work must stand on its own. If the reader has to plow through forty pages to develop any interest in your characters and setting, then that reader is not going to keep reading (your book/s, anyway).
One distinction between a short story and a longer work (and I'm stealing this from OSC and possibly mangling the idea in the process) is that in a longer work, you have the time to dip into the surrounding themes and worlds and plotlines and so forth: You can spend a little time (provided the reader feels impelled forward) describing the world or pursuing the actions of a secondary character.
By contrast in a short story you have to limit yourself severely in what additional elements you bring in. If you add long technical explanations, follow secondary characters, delve deeply into cultural differences, or otherwise spend any significant amount of time on non-central issues, your story gets too long to be called a short story.
If you would agree that a story involves introducing a primary tension at the beginning and resolving it at the end (and perhaps not everyone would agree with that), then this suggests that to keep a short story short, you have to focus on that primary tension and ignore the other stuff that comes along unless 1) it really helps the story along, and 2) you can dip into it without adding too much length to your story.
Hope these thoughts are helpful.
Luc