The rejection resubmission

A blog-reader asked me to say something about: "Resubmitting to agents when you've had a 'rewrite and send it back' kind of reject. What do you put in the covering letter?  Do you say what you've changed or just let them see? How much of previous correspondence do you mention? If you want to give them first shot and a reasonable amount of time to read but don't want to leave it forever what's the