Hey Nifty-Chicken,
How about offering static-cling window decals? If they are applied inside the car, they will be safe from car washes. Also, they're easy to remove if someone wants to change the slogans from time to time.
I don't know what all is involved with getting the bumper stickers/decals made, especially in terms of expense--but I'd like to see all the popular choices used. We have all kinds of people at BYC, so it would be nice to have all kinds of slogans.
Stephanie