Rejections, Revisions

We've all read about successful authors who racked up 100, 200, or more rejections before they were published. Some of those rejected books went on to become best sellers or even win prizes. What I've wondered is how different the book the first five agents rejected is from the version that finally sold. How much revision do you do, and when?

If you send ten query letters without a bite, what do you do? Revise your query? That makes sense if no one has even seen the first chapter of your manu...