1. Visibility is everything. I double up on lights for winter with a combination of dynamo light on my bike and a helmet light (with both front and rear lights) for added safety. I also wear a high visibility jacket with reflective fabric.
2. Studded tires! Keeping the bike rubber side down is the only way you're going to get from A to B. If you are from a climate that is wet in the winter I’d be recommending wider tires with an inverted tread (rubber side down, right?).
3. Layers are always best. Once you’ve figured out how to not get run over with lights and keeping your wheels rolling with studded tires, then you need to be thinking comfort and warmth. Several layers will do a great job trapping air as well as being super versatile in regards to adjusting the layers according to the current temp. In Minnesota the temperature can swing 30 degrees on a pretty regular basis!
4. Wear a helmet. When the possibility of slipping and sliding is very real, don't forget to protect your head from any icy falls. Plus, covering the top of your head will help keep you warm!