Motion detectors are available in cute forms that make animal sounds. Others take the form of lights that come on when someone passes by or sprinklers with swivel heads that send out hard squirts of water aimed at the triggering motion. These aren't harmful in themselves, but they might disconcert a potential invader enough that he will move on to another house. RCW Nurseries urges customers to take a good look at their outdoor lighting at night. If none exists, consider installing some.
Darkness is probably the single most inviting element to a potential burglar.
Security lighting doesn't have to be industrial-strength. It can be an asset to the landscape as well as a protective device. Spotlights not only illuminate, but also create dark areas. Most landscaping lighting companies are as expert in security setups as they are in the aesthetics of their designs. If you do it yourself, be sure to view the finished illumination with an eye to invaders as well as to your own enjoyment.