"A dog. Well-lit exterior. Nosy neighbors. A car always parked in the driveway. Lights on in the house. Anything that would normally lead someone to believe that there is someone home."

It should also be noted that outdoor security lighting is a different beast from general purpose or decorative lighting. Most homes have very patchy exterior lighting, with either plenty of glare or plenty of shadows. If it is known the house isn't occupied, poor lighting will do little to prevent someone from crawling in a basement window then walking out the front door.