Alarms can be a good investment, especially if you have many valuables in your home, or live in an isolated area or one with a history of break-ins.
Check with several companies before you buy so you can decide what level of security fits your needs. Do business with an established company and check references before signing a contract
Learn how to use your system properly! Don't "cry wolf" by setting off false alarms. People will stop paying attention and you'll probably be fined.
Some less expensive options, a sound-detecting socket that plugs into a light fixture and makes the light flash when it detects certain noises, motion sensing outdoor lights that turn on when someone approaches, or lights with photo cells that turn on when it's dark and off when it's light.