My mom had this one on her '69 convertible Camaro:
"Back off or I'll flick a booger on your windshield!"
Crass, I know...but we enjoyed it.
And remember those "Baby on Board" signs? I found on that said "Ex-boyfriend in Trunk." What made it really fun was the gaudy tie that I hung out of the trunk. You'd be amazed how many people waved me down to say, "Hey! You have something hanging out of your trunk!"
My response: "Did you read the sign?" They'd wait while I pullled away.