To screw thieves from stealing you vehicle registration plates you can get special anti theft, one-way screws. You can get them from auto and hardware stores and some local divisions of state or territory police services/forces will provide them free of charge or for a small fee. Contact your local law enforcement agency to find out more. There are a few different sizes of screws so you will need to make sure that they will fit the make/model/year of your vehicle and to install the only thing you need is a flat tip screw driver.
A little bit of this, a little bit of that and a whole lot of sewing :D