Reportedly, 90% of burglars say they avoid homes with alarm systems.  If they encountered alarms, they took off.  While there are many alarm system options, consider which level of protection makes the most sense for you.

Level 1 - Exterior Deterrents.  Trim shrubs away from doors and windows, and ensure you have adequate exterior lighting.  Yard signs advertising alarm services are good deterrents, even if there isn't an active system in place.  When you are away, keep lights on inside as well as outside the house.

Level 2 - Protect the Perimeter.  Secure doors and windows with appropriate locks and deadbolts.  Avoid windowed doors which can allow a burglar to break the glass, then reach in and open the door.  Add a security system which alarms doors and key windows, such as basement windows.  Door chimes are also helpful during the day if you have small children who may open the doors and go out unsupervised.

Level 3 - Interior Safeguards.  This layer adds protection in the event that a burglar circumvents other deterrents.  Examples include motion sensors and broken-glass detectors.  Motion-activated cameras and closed-circuit surveillance may be desired for some homes. 

Keep in mind that centrally monitored alarm systems not only provide assurance of prompt police attention, but usually provide savings on your homeowner's insurance.

And of course, all these systems are only effective if you turn them on!