A couple biiiiig dogs usually do the trick, but since our two dogs Red and Blue are just wimpy little couch warmers we have to resort to other methods to deter theft.
Like this bumper sticker that pretty much tells those would be thieves it just wouldn't be worth the hassle to smash our windows or gouge out the lock with a screwdriver.
Unless you want a McDonald's ketchup packet and a couple napkins, a dead AA battery, one sock, hair elastics and the like, there ain't nuttin worth stealin' in our redneck truck either!