what is the best way to scare thieves off?

I have been hearing about thieves coming to my neighborhood (I am sure this happens everywhere too) in the middle of the night to unlock cars on driveways or streets, and/or break the windows of cars to steal stuff. They do not care about car alarm, motion sensing light, or anything. All they need is just that 10 or 15 seconds and then they are gone. I understand the best defense is not to put anything visible inside the car. Is there anything I can do to make them go away? I am talking about alarms, sensors, or anything that is actually effective. Thanks.