CCTV
No burglar wants to be captured on camera - this is a huge deterrent, and probably the most effective deterrent against intruders.
An automated system can be connected to your video or DVD recorder - when a PIR detects movement when the system is set then cameras pick up the image of the movement and it is automatically recorded.
Linking it to a two way intercom system allows you to talk to whoever is at the door or gate.