Back when I lived in a pretty rough part of Oakland, I liked to figure that the foot-deep pile of newspapers and random items occupying the back seat of my car served as a nice theft deterrent. I imagined potential thieves seeing that level of mess and just walking on by. I doubt it made a damn bit of actual difference, but it was a convenient lie that I could tell myself to feel a little less paranoid. (Perhaps the thieves were too entranced by my license plate, which got swiped on more than one occasion, to busy themselves with the interior.)
Adam Carolla painted his stereo with a big brown stripe on it. He figured that thieves would bypass the stereo because the brown stripe made it look off and would hurt the resale value which is all thieves cared about. He said that it worked and his stereo was never taken.
Of course people ignored your car alarm going off. These things go off so frequently (due to a slight breeze, a cat jumping on the car, operator error, etc) that we’re all conditioned to ignore them. Personally, when I hear one going off, I hope it really is a car thief this time, as they’ll at least remove the car from the neighborhood. I think the states should require the auto insurance companies to add a surcharge for people with noisy car alarms and give a discount to people who get car alarms that only bother the car owner (say by sending them a text message, or displaying a message in a smartphone app).