I booked for our August trip exactly one year in advance. Price then was $777 total for 2 weeks Fullsize SUV. In November, the price jumped to over $1,500 total for the same car!
I have been watching prices for August since 2007 (we thought we would be vacationing August 2008) and I have found so far that prices are cheapest before November and jump drastically thereafter and don't go back down. At least they did not for this past summer. I am glad I locked in my price back in August.