Wear clothes that match the color of the background. For instance, if you are lying in trees, you should put on green clothes.
Conceal the exposed body parts, such as hands, shoes, neck and face. Again, if you are lying in a clump of trees, you can use the twigs to cover any part of the body whose color does not match with that of the background. Mud is another good option for concealing yourself. The benefit of applying mud to the exposed body parts is that it prevents you from being caught on infrared night goggles.
Avoid making any noises. Remain as static as possible. The aim is to merge with the background, and the background doesn't budge, so any frantic movements will be instantly detected.
Avoid the use of the skyline, even at night, as this will give away your position very easily.