I'm planning a NOBO in 2012. I need to start, for schedule reasons, at the beginning of March. The direction and start date are nonnegotiable. For my entire life, I've been very susceptible to cold. I am underweight (and that's only going to get worse on the trail) and have a very low core body temperature. Last year I climbed Mt. LeConte (smokies ~6500 ft) in late March with 20 degree temps and got a minor case of hypothermia on the way down. Granted, I was underdressed, but even proper dress probably would not have kept me warm, especially once I was wet. So... my question is this: do you veteran hikers have any tips on how to stay warm without having to carry a ridiculously heavy pack?