The single most important factor in deterring auto crimes is being selective in where you park your car. Park in well lighted and well populated areas.
Lock your car. It may seem obvious, but you wouldn't believe how many people forget.
Traffic flow is important. You want to park your car in the areas with high vehicle and pedestrian traffic.
Car alarms, can be useful, but are often ignored.
Have a kill switch installed and use a club-type device to protect your car, in addition to your alarm.
Steering column sleeves are helpful in preventing car theft where cracking the column is necessary.
Tinted windows provide extra coverage for thieves who break into cars to steal radios, air bags, or the car itself. Avoid heavily tinted windows.
Keep you car free of ALL items. Clothes, jackets, bags, boxes, sporting equipment and other items frequently left in cars, attract a thief's attention.
Thieves will break into a car to look into boxes or bags, in order to see if they contain anything of value. Even if it contains nothing, you may be left with a broken window or door lock.
Have your vehicle identification number etched in your car's glass. This deters theft. Re-tagging a stolen vehicle is too expensive if all the windows need to be replaced.
Any one of the above methods is no guarantee that your car will never be stolen or broken into. However, using as many of these devices as possible, makes a "car shopper" more likely to pass up your vehicle.