OK, since no one else has responded to this one (and I'm dying to see some other ideas!), I'll tell my one rush shirt that sticks out in my mind.
A fraternity on my campus had dwindling numbers and little recognition on campus. On the front of their rush t-shirts, they had a rising sun coming up over their letters, and on the back was written "Vision is more than what is presented to you." It may sound kinda cheezy, but their numbers doubled last year, most of the men saying they had seen the t-shirts and wondered what it was all about.