a-hole fans, and obviously Michigan is no exception. I always say "Welcome to Ann Arbor" very sincerely and usually get a positive response. They know they're in enemy territory and expecations start pretty low, so welcoming them like that comes across classy.
I'd wager that simple acts of civility, a "welcome", and maybe a beer or two generate most of the 'positive fanbase' comments across the nation. It doesn't take much.