When I get an idea, the old way of keeping it was to jot it down on a steno pad, but after a day or two I'd look back at it and go, what the hell was that supposed to be about? Nowadays I'll sit down with a new idea and hammer some of it out on the keyboard, then use that first scene as a proverbial red string around my finger to remind myself later what I came up with.
This brainstorming sit-down usually averages out to about six hundred words of rough prose. Beyond that I'm stepping on thin ice without having mapped the rest of it out, so I set it aside. The trick is coming back to it later and, in the light of day, deciding if it's an idea worth sticking with.
Currently on deck is a story about butlers. It's turning into what would happen if Fight Club and Mrs. Brown had been the same movie, and is what happens when I listen to nothing but fiddle reels on my morning commute.