i have been told that the heating pads that you stick to your back and skin work wonders for cold weather stand hunting. i have never tried it but it makes sense place it at the small of your back where your kidneys would be and it should warm up your blood-flow.

as others have said layering. i do one compression layer, a layer of insulating material preferable a wool or fleece material. the outer layer needs to be windproof and waterproof preferably and of good quality. this always keeps my body warm but my feet always seems to get cold no matter what.