Your question is answered here, and it's (most likely) coming to a bookstore near you.
So if you want to walk away with a prize from the American Heritage Dictionaries (and have the vocabulary-chops to do so) I'd call your favorite local bookstore and ask them to participate sometime during National American Heritage Dictionary Define-a-Thon Week. It has to happen during the official week for you to get a prize … otherwise you'll just get a certificate [PDF] and the joy of winning.
Has anyone participated in one of these yet? I really want to see one. I guess I'd be disqualified from entering, though. 🙂