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Layer your clothing, you sweat; you die. Do not bring cotton at all. Do you have a winter tent, aka no mesh?
Alcohol stoves are your friend in really cold weather. Isobutane fails at really low temps, alcohol always burns. Get a solid container with a strong lid for fuel storage.
Get a new, small water filter specific for this trip, keep it next to your body the whole time, if it freezes it's ruined. There's nothing worse than diarrhea frozen to your ass cheeks.
Bring hand warmers. At least 2 per day. Do not use them while on the move, they should be for camp comfort. At night heat water and fill a bottle, then sleep with it in your bag.
Get down. At least a down jacket. You want something 800 fill power or better. If it's less than that the item will weigh more and not compact down as well. I don't have any, but lots of people seem to use those down booties.
