Do you haft to have front and rear plates? 3M TA3 is a classic,but it works better on the front. That way as you come up in the rear view all they see is EAT ME.
you could try to be funny and get NO TAG - which i've seen around here (I would bet it causes problems if you get a ticket)
5OHFOX (5.0 fox)
BLUBYU (blew by you)
FSTASL (fast as hell - on father-in-law's Ligthning)
SPDRCR (speed racer...might be a bit ricey though)
IOWNU (I own you)
This one I see in the lot at work OMG BRB on a VW Bug...
Ok, this is going downhill fast. Usually i'm good at this, but it's not happening today. I'll just move along now...