There are two things you can do to a car that I think work the best. It won't stop a wrecker from picking up your car, but if they are that serious, it's hard to stop them.
Install a valve in the fuel line that cuts off fuel to the carb/injectors unless a (well hidden) switch is activated. The thief will start the car and drive off. After a few hundred feet, the car will die form lack of gas and they are stuck in the middle of the road a short distance from the crime scene. They will try to restart the car, but after it won't fire up again, rather than sit in traffic in a stolen car, they will just walk away. Better to have the police call and tell you your car was towed out of the street than to find it gone forever.
Another involves using a line-lock in such a way that it is activated (unless you disable it first) to actuate after the car is started and driven. The thief will drive off and as soon as he applies the brakes, they stay activated and lock the tires up. Once again, the thief is a short distance from where he stole the car and won't hang around for long trying to get it going again.
Other things like ignition cut-outs are also worthwhile to add because they are simple and cheap to do. It will probably stop the opportunistic joyrider, but you will need something more for the more serious thief.