From the NaNoWriMo newsletter -- those folks are brilliant:

"Writing a novel in a month is utterly ridiculous, an undertaking for fools and those who don't know any better. Thankfully, we belong to the latter camp, which makes us dangerously powerful writers. Liberated from the constraints of constructing a pretty and proper novel, we are free to run, naked and whooping, through the valleys of our imaginations.

In November, your story will achieve an at-times frightening force and velocity. Go with it. Write wildly, joyfully, in huge and bounding strokes. Was that last page the worst thing you've ever written? Maybe. Does it matter? Nope. All words are good words in NaNoWriMo. Follow tangents. Change directions at will. Stay loose. Make messes. Laugh at it all. You are doing something weird and wonderful here, and none of it will go on your permanent record."