I have yet to fall during the winter, despite riding on every kind of surface imaginable. *knocks on wood* A couple close calls where I tried to turn faster or sharper than I should've, but I've always been able to catch myself with a foot down.

Three rules for riding on ice, hardpack, and mashed-potato slush:
1) Studded tires are your friends.
2) Do not lean into turns; if you need to lean into a turn, you're going too fast.
3) Except for emergency stops, only brake or accelerate when going straight. You have a fixed amount of traction, and using some for acceleration or deceleration means you have less for holding on to your turn.