Tesco Car Insurance is provided by RSA Insurance Ireland Ltd.
Firstly, use good judgement, don't just park anywhere, try to park in well lit areas and NEVER leave your keys in the car and make sure your doors are locked.
Don't leave valuables in the car or in sight. Always remove the front panel from your car stereo, hide the Sat Nav, remove your mobile phone and never leave bags in sight. If you must leave valuables in the car lock them out of sight in the boot and where possible park the car with the boot reversed against a wall so thieves can't get behind the car.
Install an alarm system that has a siren, so that it attracts attention if something does happen. Use a steering wheel/brake pedal lock, steering wheel covers & shields, and even an armoured steering column collar to make it virtually impossible for thieves to steal your car. A lot of theft is opportunistic so anything you put in the way of thieves will reduce the chance your car will be stolen.
If you have a high performance or classic car consider using immobilisers and tracking devices. These will help to secure your car and could reduce your car insurance premium.