Kinda funny that you brought up this thread... I've spent most of my day at work pretending to be busy by actually plotting out a new "book" I intend to write/draw...
Basically I've just broken it down into sections, wrote a little description of what I hope to accomplish with each section, and then wrote little bullets of specific events in that section.
This is non-fiction though, so it might not help you out....
If you're going for fiction, maybe Bill Watterson of Calvin and Hobbes could help you out... Very, very, very loosely paraphrased:
"I very rarely know how the story will end when I start writing it... I just let the characters take it."