Jason N is correct,
the German Army issued wool lined kidney warmer belts to their troops in WW2 when they sent them to Russia . It works because your kidneys filter all of your blood , so warming them warms the blood . Putting a heater on the back of your neck will have a similar effect , though not as effective .

Kristen ,
try strapping a pair behind your knees with Ace bandages to warm your legs and feet .