well living in one of the worlds coldest counties I couldn't help thinking about what I wear. Grahams woolen uderwear is a very good Idea.

Anything synthetic starts to smell bad if used on a hike over several days,wool doesn't to the same degree.

When I mountain bike in the snow and Ice over here I use this stuff it works on the old string vest theory. I can't praise it enough.

http://www.brynje.no/produkter_eng.asp
I use the Arctic version

A good tip when buying any underwear to use in the cold is to buy it a little too long in the legs and arms. If used a lot constant washing will if you use synthetic cause some shrinkage in the material (Not your meat and two veg)Also buy the top so it covers your backside and crotch. You'll thank me for it Really you will.

Oh! Almost forgot Thumb holes in the arms are great so the shirt doen't ride up the arm leaving exposed wrists

Be safe in the cold