I heard Laura Lippman speak a few months back, and she told about getting the idea for her newest book, What the Dead Know. It was a true story that sparked her idea, a news item of the sort that we hear every day. But sometimes that item takes root in your brain, and it doesn't go away until you write it down.
With each story I've written, that happens. I never say, "Okay, I'm going to write a book now." What happens is that the story starts in my head, tapping at the windows and doors of my consciousness with fragments and ideas that make me respond with "I like that!" Usually what needs to happen next is a long drive somewhere, alone. I have a tape recorder and I talk to it, letting ideas flow freely. Some work, many don't, but it's part of the process to let plot elements rise or fall on their own.
It helps sometimes to talk to someone else, and my sister is good at getting me past the mental roadblocks. "Could she have a stammer?" she asks when I want my heroine to be reluctant to speak out. Of course she could. It's perfect.
After that it's work, getting it down on (electronic) paper, working out the kinks, and prettying up the wording. But that first idea, the one that says, "Yes, this one is worth the effort", is thrilling in a way few other things in my life are. And the best part is, you never know when the next one will arrive.