PukingMoney's excellent presentation at DEFCON 21 has some tips on how one can (legally and illegaly) obfuscate your plate.
https://www.defcon.org/images/defcon-21/dc-21-presentations/...
It's a really great primer on a number of issues concerning free travel in the USA and what law enforcement can and cannot do. Also covers other possible ways that the government can track your vehicle (toll transponders, tire sensors, etc). Read the whole thing when you have time.