There are several proven, low cost techniques that can be used against thermal/infrared imagers. But none of them last an extended period of time. While they are not fool-proof they certainly do work well enough to frustrate the US military in Afghanistan.
The first method being used is for men to lay down in a small depression in the ground and cover themselves with a heavy wool blanket every time they hear an engine overhead — be it a helicopter or the lawn-mower whine of a drone. This technique is well documented based on visual surveillance of groups of Afghans. All they did was to prop up the edges of the blanket with twigs to let their body heat escape around the edges of the blanket. This disrupts the pattern of the human body such that there is a warm spot but it does not look anything like a person.
A step up from this is to use one of the Space Blankets available on the market to do the same thing. They reflect heat right? If you have one of the heavier ones laminated to a ground cloth they can be used over and over again. In fact there is a poncho version of the space blanket that is very effective for this purpose even if you are moving about.
For shelter areas you can use mass (think of adobe houses) to diffuse the heat signature such that while the whole house will glow a bit it is impossible to see inside the house. Similar methods can be used in the field. For example, when digging a fighting position always use overhead cover if possible and pack the overhead cover with dirt or stones. This, combined with the use of blankets or tarps to screen the “windows” will render most thermal imagers unable to track you.
Finally there is always deception. In one exercise with a law enforcement team, we created a large number of “false” positives for them to track using small candles and Mylar balloons. Since aluminumized Mylar is the primary component in the space blankets, if you direct a small heat source onto the exterior surface, the whole surface will reflect the heat source and appear to be a large heat signature. (The aluminum can also be used to fool radar — the balloons can be strung in a line so many feet or inches apart — and as long as that distance apart is close to the wavelength of the radar beam the balloons appear to be one large solid target. I found this out many years ago when I was living very close to the largest helicopter base in the Argentine army just outside Rafael Castillo. We could float a string of balloons and get an almost immediate response …)
Another trick is to place a small candle (120 hour candles work well for this) under a piece of metal about the size of a hubcap or 20 gallon drum lid — as the candle burns it heats the entire surface without any hot spots but rather fairly even heating. To the poor chap looking through the thermal imaging gear now sees a heat source that is about the same size as a person’s head and is not moving the way an animal would when a helicopter is overhead. So it has to be checked out.
Then there is my favorite one. Simply lay down to take your nap amongst a herd of goats or a flock of sheep. – Dr. D.
JWR Adds: Another method of infrared camouflage is to encamp in an area with numerous natural hot springs and pools. Since these are often associated with natural salt licks, these areas tend to attract wild game. Thus, not only will the hot springs themselves create distractions, but so will any deer (and similar-sized hoofed animals) that are in the area.