I have a potential novel in my head at all times, and each night I work on it, laying there in the dark with my eyes closed. Characters get developed, plots start to form. Each night I add the smalled of details, really giving the people and the settings a real feeling. I do it so often, it becomes almost like a memory. I work on it until I fall asleep--some night sooner than other depending on the amount of caffeine I have had.
If I am not in the mood for that, I put my ear buds in, and turn an audiobook on with my iPod. Works like a charm.