My only experience of hunting in the extreme cold was in Sweden, where it got down to a relatively balmy -10C
I would echo the points above:
> if you don't have a Peacock warmer then the chemical warmers are also very useful. 5 each day is a good idea - one per hand, one per foot and one in the hat
> if you're sitting down waiting then one of the self-inflating cushions can be a good idea. In Sweden they then cover them with fur, as you can lose a lot of heat otherwise
> layers is definitely the way to go. I like merino, then one or more fleece layers, then an outer shell.
> take a small rucksack with you where you can store a spare fleece or gilet and the other stuff below
> take one of the fleece neck warmers, which you can pull up over your ears if the wind really gets up
> for the same reasons take a coat with a hood
> thin gloves, thick gloves, then mittens will keep you warm
> don't forget to stick some spare socks in the rucksack in case your feet get wet during the day
> cut the legs off an old pair of wellies and then cut right up the back to give you a rubber sheet that you can stand on or rest your feet on to keep them insulated from the ground. You might be surprised how much difference it makes, and the rubber can be rolled up and stuck in the rucksack when not needed.
> buy some of the small Dairy Milk chocolate bars at the airport on the way out - enough for one each morning and one each afternoon. It is important to keep the energy going and you burn off a lot of calories sitting there shivering
> take a small thermos - I like coffee, soup or Bovril. If you think there might be a lot of waiting then consider something like the Jetboil