Do not leave valuable items in your vehicle (GPS devices, laptops, PDAs, cell phones, MP3s, wallets, purses, money, credit cards, etc.), and do not leave items with personal information in your car. If unavoidable, remove everything from plain view, including the garage door opener, but do not move your valuables to the trunk while in public view.
Do not leave vehicle doors unlocked, and do not leave windows or sunroofs open.