I liked the wanted system in Driver: Parallel Lines. If all the cops see is your car, they're chasing the car. Ditch the wheels out of sight and the cops aren't after you anymore. If they see you get out, or they see YOU doing something they're after you.
XBL Gamertag: JWRebholz
PSN ID: jwrebholz
...No, I'm not very creative when it comes to this sort of thing.