Lots of good suggestions in this thread.

My additional small comments:
1. Dont wear cotton. It soaks water (snow, sweat) like a sponge, and
doesn't dry fast. If you get stuck overnight in wet cotton, you will
have hypothermia.
2. Wool is good. Insulates when it is wet. Doesn't have as strong a
"stink memory" as polypro and spandex.
3. Always have a good waterproof shell. You can go a long way with a
thin, but good, shell. Especially if you are in coastal mountains,
where the snow has a much higher water content.