A guy once gave me a tip and on cold days it seems to help me.. I put some hand warmers down near my kidneys.. It seems like the blood circulating through with all that heat right there really helps keep my core warmer...
I tried the back ache packs that actualy strap around ur waist and have the heat pads for ur back work great.. Put those baby's on ur kidneys and its like a little radiator

Whether it really works I don't kno but it does in my mind haha