The product that has worked best for me over the years is the Little Switch, from Flaming River ($30; 800-648-8022; www.flaming-river.com). You splice this heavy-duty device into the truck's electrical system. When activated, it completely disconnects the system from the battery and essentially turns the truck into a 6,000-pound doorstop. Its special feature is a removable key that you take with you. Even if the thief locates the switch, he can't deactivate it.