Always park your car in a well lit area. Have the key in your hand when coming back to the car. You will be able to enter quickly. Shopping centers are well known for carjacking. Be especially careful when shopping alone.
Mace is often very effective in warding off perpetrators.
Keep all valuables and shopping bags in the trunk when going in and out of stores. Walk as fast as possible when going to and from the car. Thieves are always looking for slow walkers because they feel that they are not alert or concerned with their immediate surroundings. Make sure your car is in good condition and well maintained at all times to prevent breakdowns. This way you can make a quick escape when necessary.