One of the first things I did when I moved into my house was install several exterior lights.

Exterior Light

This was done partly for looks but mostly for security.  Three of the lights had sensors built into them (the two by the garage and one by the front door) so that they would turn on automatically when the sun went down.  This illuminated my house at night, making it look occupied even when I was away.  This may sound minor but light is a powerful deterrent to crime.  Crooks have fewer places to hide at night where there is illumination, which is one of the reasons why streetlamps are installed in urban areas.

For around $300, I was able to purchase these lights and get them installed.  The energy costs are low since I use 15-watt CFL bulbs instead of incandescent ones, plus I haven’t had to replace one in the four years they have been operational.  Its a small investment that provides a large benefit to my home’s security.  So keep the lights on, its a great way of hardening your home!