So if you are reading this post, it would mean you are already considering a ride to work on a very cold day. I am rooting for you! Get out there and do it! Here are just a few things for you to consider.

Dress for It 
Long Underwear - don't leave home without it.
Ski Helmet - best Christmas present I got this year.
Ski Goggles - your face will freeze without them.
Tall Snow Boots - they are ugly, but the taller the better. Leave an extra pair of shoes in the office.
Fleece Vest - the high collar will keep your face warm, and the vest will keep the mud off your jacket
Backpack - it can carry your stuff, but it will also keep you warm

Have The Right Gear
Mud Flaps - well worth the $20
Milk Crate - good for storage, and doubles as a rear mudflap
Headlamp - it gets dark really early in the winter
Taillight - I mount it on the back of my helmet

The Right Bike
Older is better - the road salt will do a number on your components... please don't use your new bike
No Clipless pedals - the ice is too slick, and the shoes aren't warm enough... don't bother
Knobby Tires - the snow and ice is slick, put on the fattest ones you can find
Low Air Pressure - even the best tires don't have much tread on the sidewalls

Adapt Your Riding Style
Stay Seated - in order to keep traction, you need to keep weight on the rear wheel at all times
Higher Gears are Better - while a lower gear gives you more torque, it will spin the tire. Grunt it out!
Keep it Straight - while learning to avoid obstacles is normally part of good riding technique, riding in a straight line is even better technique in the snow. ride right over the clumps of snow, and you will keep it upright for longer.

Be Considerate To Traffic
They Don't Expect You - remember that a biker in the snow is totally unexpected
Be Visible - they need to see you a long ways off
Its Ok To Be There - I have found it to be much more dangerous on a jagged sidewalk, than the shoulder of the road. Just keep it smooth and steady, with no sudden movements, and no one gets hurt.

Have Fun!