Obviously, the ultimate goal of having a home security system is to detect and prevent unauthorized entry. But you have to be certain that you consider all possible ways that a burglar could enter your home and choose a system that addresses each one. For example, if your system doesn't cover basement windows where a burglar could enter, or if it doesn't cover entry through the garage, you have an incomplete security solution. You can use motion sensors strategically so that you don't have to have sensors on every window, for example, but every likely entry point should be covered in some way by your security system.
If the keypad that activates your system's siren is mounted right next to a door or window, a burglar entering there will immediately know the source of the alarm and may attempt to tear it off the wall. If the keypad is integrated with the siren or if it's mounted adjacent to the siren, a burglar can make quick work of disabling the system. Another reason to avoid mounting a keypad where it's visible from the outside is that a burglar may be able to look through a window and determine that the system isn't armed, in which case, he'll proceed to break in.
Motion detector placement is very important to the proper functioning of your home security system. If you should rearrange furniture so that a large potted plant, shelving, a home entertainment center or other objects block the motion detectors, they can't work properly. It's also a good idea to locate motion detectors away from microwave ovens (to avoid crossed signals), as well as other wireless devices.
Don't make the mistake of thinking that because you have a home security system, you can dispense with other home security measures, such as having deadbolts on all exterior doors or installing outdoor lighting. The best home security consists of several layers, including making sure everyone in the family is committed to home security, outfitting doors and windows with strong locks, and making sure the alarm system is armed overnight and any time the house is empty.