How do you plot?

I can't remember if we've ever talked about this before ....

How do you plot out your books? I know some of us are 'seat of the pants' people, but we still have to have a germ of an idea to get the book rolling. Do you have a Beginning, Middle, and End in mind when you sit down to write? Do you just have the first chapter? Or do you have the end of the book firmly in mind?

What keeps your plot chugging along -- an outline? A few scenes? Share your process....