Just make it harder for the crook to steal your car than the next guy's. First, take some common sense measures. Talk about an invitation to thieves. Even at home you should lock your car. Don't forget to lock the garage, too.
Residential areas are the site for more than half of car thefts.
When away from home try to park in well lit, well traveled areas. The 'bad guys' love dark, lonely places. Over two thirds of auto thefts happen after dark. If you're parking on the city street, turn the steering wheel sharply to the curb. Put your emergency brake on.