The opportunistic thief will usually only target a motor vehicle if they think they can get away with it, so you need to be as proactive as possible to deter them. Do not leave valuables inside the car.
It will only encourage their interest. This includes handbags, loose change, trade tools, laptops and GPS navigators.
If at home, park your car close to your house so noises can be heard and use sensor lights to detect movement.
Try not to park your car on the verge where you cannot see it or hear activity near it.
To prevent theft of registration plates consider securing them with anti-theft screws available at most larger hardware stores.