For the really cold (under -10F) days, I wear a home-made mask, hat, helmet, long sleeve t-shirt, fleece jacket, and waterproof shell. Long undies and flannel/fleece lined jeans and waterproof non-insulated snowpants. For feet, I ditch my insulated hiking boots that fit in my clips/straps, in favor of my ice climbing boots.

My tips:

1.) Layers are awesome, but don't be afraid to stop and shed if you start sweating.
2.) Learn how to swap out a blown tube as fast as possible, carry a couple extras, patching is near impossible in freezing temps.
3.) Pay attention to any exposed skin (see changing tire fast). Frostbite is a btich.