I just wave when they yell. If I yell anything back, it's "hi" in a real happy voice. there was some guy that yelled "get off the ****ing road" as he drove past, and then yelled "get off the ****ing road" again from his driveway. I yelled "hi" in my happy voice, and he finally said "hi" back. Made my day.
I have been tempted to yell silly things at my friends when I see them riding, but that would only be funny if motorists stopped doing it so much.