Know Your Neighbors
Knowing your neighbors is a cheap form of home security. Well-acquainted neighbors are more likely to call you or the police when they see something fishy.
If they know you're out of town, for example, and they see a furniture delivery truck parked in your driveway, hopefully they'll call in the suspicious behavior. Piled up newspapers on your driveway, promotional fliers left on your front door or even packages sitting on your stoop are all signs you're away from home. Ask a neighbor to pick up these things so that anyone watching the neighborhood doesn't get tipped off that you're out of town.