Here's how one guy does it.

<blockquote>Coulton realized he could simply poll his existing online audience members, find out where they lived and stage a tactical strike on any town with more than 100 fans, the point at which he’d be likely to make $1,000 for a concert. It is a flash-mob approach to touring: he parachutes into out-of-the-way towns like Ardmore, Pa., where he recently played to a sold-out club of 140.</blockquote>

http://www.nytimes.com/2007/05/13/magazine/13audience-t.html