1. Always lock your car and use an alarm that has a status indicator LED light that is visible and flashing. Even better, you may want to consider installing the Club, a Passive Immobilizer, shock sensors, and/or a GPS Vehicle Tracking Device. These devices tend to be more effective than only using a car alarm.
2. If you cannot park inside a securely monitored / manned parking area, then try and seek areas of high visibility near a busy roadway for example. If possible park your vehicle where you can personally observe it, or close enough to check on it from time-to-time. This is not always realistic depending on the circumstances. If parking at night, try and park under a well lit area or near a motion light.
3. When your vehicle has been parked in an unprotected area for an extended period of time you may want to check it for tampering. Visually inspect your entire vehicle from front-to-back by walking around your vehicle. Cases have shown the simple act of checking a vehicle on a frequent basis to be a viable deterrent against theft or vandalism. It sends a message to potential thieves that you are attentive and a less convenient target. Suspects frequently case parking lots well before the actual theft happens and your presence is likely to get noticed, thus increasing the chances that they will simply move onto an easier target.
4. Never leave any valuable items inside your vehicle.
5. Never leave any personally identifiable information or documents inside your vehicle. Carry all checks, bank documents, vehicle registration, IDs, license, permits, or other documents on your person. It's a good idea not to leave your home's garage door opener (remote) inside your vehicle or any keys.
6. Lock your gas tank, trunk, and the hood of your vehicle. Make sure all windows are completely closed.
7. Keep your vehicle well-maintained and clean. You don't want your vehicle to appear to be abandoned.
8. Call the police if you observe someone breaking into your vehicle. Never attempt to approach anyone yourself.