Keep the interior of the car messy. Bags of potato chips on the floor, pieces of lettuce and tomato that are dried up over shifter, empty cups everywhere, cig butts in ashtray, drop a few m&ms between the seats and on the floormats so they melt and get chocolate on the mat, taco bell wrappers in the car for at least a week so you get a certain smell in the interior. When the thief sits in the car about to crack the ignition, the taco bell smell will make him/her say what the hell, then your car will be safe.