I have found the little handwarmers in the larger size with adhesive on the back work great on the abdomen/or chest. Think about your core area, all the blood goes through your heart eventually, keep that area warm.

Then worry about the hands, feet and head.

The rest is layered in warm clothes. I have a pair of wool pants that are awesome for keeping the legs warm and layered off with my camo covering that beats down the wind.