I start with one kernel of idea. Sometimes it's a theme, sometimes it's a particular scene or snapshot, sometimes it's a line of dialogue. I let it percolate for a while - maybe a couple of weeks - and eventually it builds up on itself. I take notes for a week or two (generally few of which end up contributing to the eventual novel) and then it develops itself once I start writing it.

This is generally how I do stuff for NaNo - I only start casting around for ideas a week or two before November, knowing that something will jump into my brain in time.