There are a couple easy things you can do to stay “rubber side down” in the snow.
Seriously, slow down, slow way down. The more snow, the slower you should be going. Not only will this give you more time to stop, it will also give you more time to go around that ice (you saw that ice right, because you slowed down…right?), dodge that pedestrian, and avoid that asshole in the cab trying to pick up that pedestrian.
Ride in a straight line
If you find yourself on particularly slick road (maybe even a patch of ice), don’t panic. Suddenly slamming on the brakes, or trying to turn sharply to avoid the obstacle is a sure way to end up with your ass in a frozen puddle. If you find yourself on ice, stay in a nice straight line, slow down gently and gradually (shouldn’t be a problem if you follow the tip above), and remember to put your foot down some place solid.
Know your physics
Things like utility hole covers, metal plates, white lines, white stripes, that rubber stuff around train tracks, train tracks, grates, the yellow line down the center of the road…all have something in common. They are made of different stuff than asphalt. As such they all heat up and cool down at different speeds, meaning one thing ICE. All of those structures are more likely to have ice on them. Basically treat them like they have ice on them, slow down as you cross, ride in a straight line, try not to hit them on turns.
Sit and work it
A friend of mine is fond of saying “sit and work it” when I try to get out of the saddle to go up hills. Regardless of if sitting down while hill climbing is a good idea or not, sitting down on the saddle while riding in the snow is a good idea. The added weight on the back wheel will help to control the bike, and because you are sitting down the overall weight of the bicycle will be lower, thus giving you better balance (due to the lower center of gravity).
Ride in the road
In the winter the sidewalks and bike paths become a hellish nightmare of ice and snow. It’s really not even worth it to even think about riding on them (even the Minute Man is poorly plowed). Get out into the road where the large salt filled trucks have cleared you a path. This can sometimes mean riding in that little gap plowed out by the car wheels in front of you, or it might mean riding several feet further into the road because the snow plows have filled up the bike lane. Either way, be visible (lights, reflectors etc), and be confident.