It's simple. Go to the club where you hope to play. Go early and put a little "sumthin'" in the scheduled band's food or drink while they're soundchecking. When they become unable to play, tell the manager that you have a great band that could be there in ten minutes. Tell him you'll play for less money and half the bar tab. Then gather up your posse and use the other band's gear to play your show. Play loud, you want to make sure they can hear you greatness. Be sure to drink a lot, it'll improve your stage presence. And hit on every attractive woman in the place even if she is the owner's wife. She'd going to dump him anyway.
That usually works, but somehow we never get asked back for a second booking.