Stephen Kings says writer's notebooks are the best way to immortalize bad ideas. He says a good idea is one that sticks around.

I admire his talent and his apparent ability to keep a lot in his head, but I respectfully disagree. I for one would be lost without notebooks in which I write character sketches, freefall passages, snippets of dialogue, descriptions of places, ideas (good and bad) and so many other things that I don't want to lose track of.

Do I use all of it? No, but I use a lot of it to good effect.