I'm not one of those writers who has a plethora of ideas. In fact, I'm more like those author characters in Stephen King's movies/books who are always worried that they've run completely dry. So, unlike like some who have a really cool answer for this question, I have to actually sit down and mechanically assemble a plot.
Of course, for any of you who know me (think: slightly ditzier than my character Garnet), realize that the term "mechanically assemble" and Tate do not go hand-in-hand. So normally my idea process starts with a deadline, ala, "Oh s&@t, you mean that short story is due on the anthology editor's desk in ten days? Arrgh!"* Then, I run around the house talking to myself. "Okay, what was the theme? What can I do on that?"
Some time later (after the panic has worn off a bit), I sit down in a quiet place and start with what my friend (and fellow writer) Kelly McCullough and I sometimes refer to as "a problem statement." What a problem statement really is, is a phrase that describes the conflict of the story (or novel) in a concise way. An example might be: what kind of horror does it take for a seasoned soldier to abandon his duty?
Then I try to figure out what my answer to the question is. From there, I have the kernel of the short story/novel, and more often than not I can either write a plot outline/synopsis for a novel or get started on a short story.
This doesn't always work as smoothly as I describe it, of course. Often I'll get started (particularly in the case of my novels) and realize what the book is really about 2/3rds of the way through. Then, I usually have to do a whole boatload of revisions, which isn't a lot of fun. That's why, actually, over the years, I've tried to do as much thinking about a novel/short story BEFORE I start writing as humanly possible (given the usual constrains of the looming deadline.)
How about you? Where do you get your crazy ideas? What's your process like?
* Real statement made about twenty minutes ago.