Sometimes, you just need to access a room that you don't have access to. Maybe you want to leave a surprise on your spouse's desk. Perhaps you want to play a harmless prank on a friend who is paranoid enough to have a motion detector in his room. Fooling a motion detector is not a simple process, but it can be done.
Learn how the motion detector works. Motion detectors have sensors with a semicircular field of view and look for heat waves radiating from moving objects. After being tripped, the sensor will alert the homeowner. It may switch on a light for a certain period of time or alert a security system or start a siren.
Trip the motion detector on purpose until the homeowner switches it off. Motion detectors can be fooled easily by blowing leaves or tree branches, and a lot of people get frustrated by the intermittent light and simply shut the detector off.
Reduce the amount of heat you're giving off. Motion detectors that search for infrared heat will be triggered by a person-sized spot of heat. Use this to your advantage. Try soaking in a cool bathtub for a little while to reduce the heat you give off. See if you can train a dog who has a much smaller heat profile to get into the room and do your bidding.
Introduce a masking sound. Detectors that listen for sounds can be fooled when you broadcast white noise into its range. When the detector is coping with the white noise, the sound of your breath and footsteps won't trigger the alarm.
Figure out the paths of the detectors. You may have done this in a video game. When you know where the detectors are looking, you can sometimes wriggle your way through a path just out of range of the detectors.
Move very slowly. Older motion detectors can be fooled this way. It may take a little practice, but you can get where you're not supposed to be by moving too slowly for the device to detect.