Your best bet is to get a moiston wicking legging to go underneath. It will provide muscle compression and support. Nike and Under Armour make tight fitting leggings that work great at moisture wicking. They have different materials that are designed to keep you warm/dry or cool/dry. I am almost never cold on a mountain so I go for the best moisture wicking properties + cool. Every snow shop has base layers that are designed for all kinds of conditions from mountain specific brands.
I wear a pair of Under Armour cold gear, and depending on how cold it is, I have a pair of Modrobes I ripped the whole crotch open on that I wear under my snow pants to add an extra layer, even though the modrobes are really light they do the trick to keep your legs warm. (for those who don't know what Modrobes are, they're essentially hospital pants)
I always wear a shell jacket and shell pants. This requires two layers underneath unless it's a warm day. I was nice and comfy in 0 degree F weather because of my layering. I use under armor cold gear 3.0. It's amazing.
i usually just wear like my boxers. but recently i've decided to wear an extra layer of basketball shorts. it really doesn't matter on the west coast where temperature rarely ever hits below -7. you'd probably have to be more conscious of layering at places where it's colder.