Like ronn and Hunting Man, I also use the back wraps with heating pouches. Another item is the foot heaters (they look like thin inner soles)that work on the same principle. MOst of this stuff will produce heat for an entire day. I think the clothing with battery power heat run out of juice long before that.
Last year I invested in a complete outfit - underwear, shirt, jacket, etc. - of the Medalist brand of clothing that has silver threads woven in. This material reflects a lot of heat and seems to be working well for me here in Montana.