Having a car alarm won't keep your car safe by itself, but it does add a level of security. Though car alarms are known for causing exasperation when they go off unnecessarily, they are a deterrent to potential car thieves. It's hard for a carjacker to get away unnoticed while driving a car with a wailing alarm.
There are many brands and varieties of anti-theft locks on the market, including steering wheel locks and brake pedal locks. Brake locks can either be locked to the brake pedal or, in a vehicle with a manual transmission, to the clutch pedal, rendering the car useless. Among steering wheel locks, The Club is the most well-known. The Club fastens to your steering wheel and makes it impossible for the car to be turned. Also, it's highly visible, and carjackers may choose an easier target after seeing the lock.
Another kind of auto theft prevention device is an immobilizer. Immobilizing devices such as kill switches and starter disablers make it impossible for carjackers to hot-wire your engine and drive off with your car. Most newer cars come with such a device, or they come with smart keys, which contain a microchip. Vehicles with smart keys cannot be turned on unless the correct key is used. Immobilizing devices can be installed on older cars.
As a final means of protection, tracking devices that track a car's location if it's stolen can be installed. In some cars, tracking systems come pre-installed. GM's OnStar system is a well-known example. However, these tracking systems often use cellular service to broadcast information, so if a thief has taken your car to a location with no cell service, police may not be able to use the tracking system to locate your vehicle. The LoJack system, while more expensive, transmits information directly to law enforcement via radio waves. Stolen cars with LoJack or similar auto theft prevention devices are recovered far more consistently than those without them.
Of course, no device or alarm system is a substitute for common sense when it comes to auto theft prevention. Always lock your car, never leave the keys inside, roll up the windows, park in safe, well-lit areas, and don't leave valuables in plain sight. Auto thieves are looking for easy targets, so don't be one.