Thermal cameras are an excellent method for detecting hidden objects. Police, for instance, can use them to detect secret compartments in vehicles because the hidden object gives off a strong heat signature. Snipers in camouflaged Ghillie suits may completely fool the naked eye, but their heat signature is instantly picked up on a thermal camera. Blocking heat signature is possible, but only for limited amounts of time. The problem is that completely masking your heat signature equally makes you stand out on thermal camera. There are alternate methods that do not block heat, but can confuse a thermal camera.
Wear a neoprene wet suit to mask your heat signature. This method was proved on the Discovery Channel program "Mythbusters," but their experiment also showed that the effects were short-lived. The suit quickly heated up the longer it was worn and lost its concealing effects.
Attach a medium-sized pane of glass to a long metal arm and hold it in front of you as a shield. When the glass is aimed at the camera, the infrared beams are filtered and cannot detect the human heat signature. This method was also proved on "Mythbusters."
Move in areas with busy thermal activity, especially around industrial areas. The heat given off by buildings and cars provides you some extra cover from thermal cameras. Hide near rocks, which absorb heat during the day, if you're out in nature. The extreme option for this step is to start a fire, so that your heat signature is no different from the burning objects around you.
Use a thermal blanket to cover your heat signature. These are quite expensive and it's important to carefully conceal yourself so that you don't create an obvious cold spot on the thermal camera.
Break up your silhouette using sticks and other objects. This is a basic tactic of camouflage, and it works well with thermal imaging too. Your distinct human form is broken up into odd angles, while still providing enough heat signature to avoid an obvious human-shaped blank.
Methods that do not work for blocking thermal cameras, as proved by "Mythbusters," include covering yourself in mud and raising the temperature of the room to 36.6 or 37.2 degrees C (the average human temperature).