Lots of good advice. I'll add my $0.02 from snow camping.

A good pad is essential--closed cell foam (ensolite) is the best for insulation. If you like a more comfortable air pad, make sure it is appropriately insulated or put a closed cell on top of it.

Dry wool, fleece or acrylic socks but make sure they are loose fitting otherwise they will restrict circulation and make your feet colder.

Always sleep with a beanie. Always.

If your jacket is heavy and you put it on top of your sleeping bag, it may actually compress the insulation and make you colder. Think light, fluffy thoughts!

I always toss a hot water bottle down in there. Make sure the lid is on tight and it's covered, as mentioned.

Do eat a hearty snack before bed. Don't eat/drink things with caffeine--it makes you need to pee, keeps you awake and is a vasoconstrictor (less blood to extremities=colder).

If you're really chilly, 20-40 push ups in your bag will often cure it! (and can be done quietly enough to not wake your tent mates)

Gross points: If it's really cold out, use a pee bottle, preferably one with a wide mouth, tight sealing lid and unmistakably differentiated from the tequila bottle that was passed around the fire earlier. Once used, keep it in your sleeping bag. No point putting that warm liquid out in the cold! Gatorade bottles get my nod for this.

Best things about camping in the snow are no crowds, no bugs and no dirt!