It depends how cold it is. i always wear a couple of T shirts, a fleece and my jacket. If its really cold i wear footless tights and thicker socks. I used to wear two pairs of socks then realised thats how you get blisters.
Thermals are good - I wore them last year with a mid layer on top, then ski gear. Mind you, don't let anyone take photos of you wearing them - they're unflattering to say the least. Our offspring have been merciless since they discovered pics ...
Def thermals as bottom layer. Bought my daughter peter storm ones from Millets very soft and light and comfortable. will be wearing mine but absolutely no one will see them!
Last year we wore on coldest day - thermal top and legs, roll neck, fleece jacket, warm hat + jacket hood up, goggles neck warmer, thick socks - and yes it took a major decision to decide to go to the loo! but it was brill! we were beginners so slow snow plough not as energetic as the expert ski dudes!