My fave is one I saw in town here, it reads:
"For a small town this one sure has alot of assholes"
I thought that might look nice on the back of my cruiser but somehow I don't think the Chief would approve. No sense of humor.
In the late 70's/early 80's, it was popular to go to Greensboro College and get a sticker, remove Greensboro and just put College in your back windshield ala John Belushi.
How about a "practical joke" where you slap them on the bottom of your buddies bumper?