If you park your vehicle in a garage remember to lock both the car and the garage, consider extending your house alarm to cover your garage if it does not already do this.
Remove the ignition key every time you leave your vehicle for short periods of time, perhaps when you are filling the car up with petrol or popping into the newsagents. It only takes a few seconds for someone to jump into your car and drive away.
When parking in a public place, leave your vehicle in a well lit and well spaced location. The location should preferably be visible to passers by. If you plan to leave your vehicle in a car park, choose one that is security controlled or managed by patrols.