Alarm systems can significantly enhance security if installed and used properly.
Phony "Warning: These Premises Protected by the Secure Alarm Company" signs are usually not effective, a serious burglar need only look in the telephone directory to see whether an alarm company exists. Most companies won't let non-customers display their logo and the burglars know that too. There's no harm in using such signs, but don't "expect" them to provide any protection.
If you are intending to have an alarm installed, always use a established company that is registered as an approved alarm installer with the local police, to install, repair, or service an alarm system. While licenses do not guarantee honesty, it does indicate that the supplier has registered with the police, and has met the specified minimum criteria for your area.
Local alarm systems (those which sound only on the protected premises) are much less effective, especially when councils limit the time for which the signal can sound to avoid nuisance disturbance of neighbours. If you invest in such an alarm, you are counting on conscientious neighbours or passers-by to call the police to respond. Having the signals from your alarm system monitored by a secure central station assures that you get the protection you pay for when you install an alarm system. Other systems can dial your number at work or your mobile phone and inform you of an alarm at your home or business.
Remember the best alarm systems rely on phone lines. Check where the lines connect to the house or business. If they attach at the ground floor level, they should be routed through metal conduit to prevent them from being cut.
The best alarm systems are those which combine perimeter and interior detection devices. Every door should be protected with some type of switch device. Covering every window (or even every "downstairs window" in multi-story houses) can get very expensive very quickly, your decision should be guided by local experience with criminal entry through windows, if that's a common occurrence in your area, window protection is probably advisable. If not, interior protection may be adequate. If window protection is needed, glass-break sensors are the preferred device.
Interior devices cover a volume of space, and are typically wall-mounted. The least-susceptible to false alarms are passive infra-red sensors (which are really thermostats that detect the presence of a human intruder by comparing the 98.6 degree body heat to the usual ambient temperature in occupied space which is typically between fifty-five and seventy-five degrees). Pets, air currents, rodents, and other similar sources will not set off passive infrared devices, and thus generate false alarms. Don't forget to provide coverage for any loft access in exposed or semi-exposed locations (like the garage). Consider installing one or more panic buttons at fixed locations, or obtaining one or more such devices which can be worn on a chain or key chain if you have members of the family with health problems or limited mobility.
You'll need to decide whether you intend to use the system when you're home, or only when you're out. If you want protection when you're at home, the system will have to have "zones" so you can arm the perimeter while leaving occupants freedom to move about.
Every system has a control panel of some type. In most security systems, the controls are concealed in a cupboard or utility room and only the controls necessary to operate the system are in occupied space. In most systems, these controls take the form of a touch-pad resembling the one on your telephone. You "arm" or "disarm" the system by punching in a code. Most users find it convenient for all members of the family or business to use the same code, but many systems are capable of multiple codes. Some systems are also capable of using arm codes with one less digit that the disarm codes, allowing you to have a maintenance worker turn the system on when they leave, but unable to turn the system off. You'll need to consider how you want to use the system before making a decision about type and installation.
Installing the keypad inside the protected space prevents any access by unauthorised persons, but necessitates that there be a delay in signalling an intrusion (at least from the entrance where the control is located) in order to allow sufficient time for a person to disarm the system upon entering. You can eliminate this delay by installing the touch-pad outside the protected area. While this does subject the pad to attack, you can minimise the risk by installing it inside the garage rather than at the front door. Many people come and go through their garages and inside installation of the touch-pad minimises any opportunity for someone to attempt to defeat the system through that device.
If you have high-value assets, there are supplemental devices available to extend alarm protection to these items. You can alarm a cupboard used as an inside "strong-room", a display case for collectibles, a safe or vault, or wall-hung artwork. Any competent alarm installer can assist in devising a means to protect almost any asset.
Alarm systems don't eat, sleep, get bored, or take holidays. They are however electro-mechanical devices subject to failure and must be tested regularly to ensure they work as designed. Set up a test schedule with your installer or alarm company, at least once monthly is highly recommended. If your alarm is a wireless system make sure it is self monitoring and will signal you if a battery is low.