Burglar alarms are not cheap. Costs vary depending on the company you use and what you want installed. Prices average about $600 for the main system, plus $40 to $60 for each window and door you want tripped.

The three main types are:

Motion Detector--the intruder is already in the house and his movements are picked up by the alarm, which then sounds. Not advisable if you own a dog or cat, or tend to sleepwalk at night. Any movement sets it off.

Impact--Any blow to your window or door (animals, birds, a child's ball, a cat sitting inside a window, etc.) will activate your alarm.

Open Door/Window Sensor -- Probably the most practical and least bother. The alarm goes off when anyone disturbs either a window or door and continues to sound until either you turn if off or its set time span (usually 12 minutes) is complete.

In deciding on a reputable company, Bill Greer of the National Burglar and Fire Alarm Assn. (NBFAA), and the D.C. Police Department suggest that you:

* Ask your insurance agent for a recommendation, and inquire about premium discounts for installation of a system.

* Inquire about years in the business, licensing, if applicable, manufacturer's guarantee, 24-hour emergency monitoring, maintenance contracts and their duration, membership in a trade association.

* Get written bids from three or four companies. (Some companies will lease their system and remove it if you sell your home.)

* If unsure of a company, check with the Better Business Bureau or Consumer Protection Agency.

* "Don't rush into it," says Greer. "Expect to spend a lot of time researching, to get a system to suit your particular situation. There are many options available."

Other reminders:

* A trusted neighbor should have either your key or combination number to turn off your alarm. Alarms are sensitive and a clap of thunder can activate them. Twelve minutes can seem interminable to irritated neighbors or needlessly called police.

* Decide in advance if you want your alarm activated by a key or a number combination. Each has advantages and disadvantages. Keys get lost or stolen, numbers can be forgotten.

* Be consistent. You must set your alarm every time you leave the house. Teach children the combination and make sure they understand that the numbers must be a family secret. For safety sake, they shouldn't be neighborhood gossip.

* Damage of the wires around doors and windows can cause problems. One well-known socialite had to call in a repairman after her child's pet pig chewed through the wires.