Travel Slowly. Allow more time for braking, and to react to changing road conditions. Slow down more than usual for curves and turns. Another advantage to not speeding is that it will hurt less if you do skid and fall off. Which probably will happen at least once over the course of the winter.
Riding on Snow and Ice As snow compacts into ice, it becomes slippery and lumpy. Riding in bare patches and non-compacted snow is safer.
Beware of Motorists I've received considerably more abuse from motorists in the winter than in the summer. Cyclists are less visible in the winter darkness, and perceived to be more "in the way" when they avoid the snow that accumulates at the sides of the road. Remember cyclists have the right to ride in a general traffic lane when bike lanes have not been cleared of snow.
Take Off-Street Trails Winter bikers in other metro area cities are jealous of Minneapolis with over 80 miles of trails, most cleared of snow as quickly as city streets are. The Midtown Greenway and the Hiawatha Light Rail trail are usually plowed within 24 hours. The major benefit of using trails is no motor traffic. Use them if you can.
Stay Warm Dress in snug layers. A base layer, an insulating layer, and a waterproof, windproof layer on top is a good foundation. Add extra layers or vary the thicknesses for different temperatures. Synthetic, wicking fabrics are preferable over cotton, which holds in sweat, then the moisture will make you very cold.
Keep Your Extremities Warm While your body will usually warm up quickly as you cycle, your hands, face and feet need protection from the cold. Your bike's wheels throw snow on your feet and hands, making them even colder. Waterproof, insulated gloves are essential or your hands will be freezing within seconds of starting your journey. Thick socks and boots keep toes warm, and chemical toe warmers are a blessing on long rides. A close-fitting hat or helmet liner keeps your head warm, and a scarf or face mask takes care of cold cheeks. Goggles, such as ski goggles, keep freezing wind and snow out of your eyes and reduce glare from the snow.
Be Visible Legally, cyclists have to use a white front headlight and a red rear reflector at night. Any sensible cyclist adds at least one more rear red light: one on the bike, and one on the back of their helmet is a good combination for extra visibility. Reflective clothing is a good choice too. Check your lights are attached securely to your bike. The first time I rode in the winter, the cold made my bike's frame shrink, but the plastic front headlight mount didn't. I had to balance the light on the brake cables to keep it pointing ahead until I could get a screwdriver to tighten the mounting.
Choose Your Ride and Look After it Most bike commuters favor an old bike with wide, mountain bike style tires for the best grip, although any bike will be fine on plowed streets. Many riders consider studded tires vital for safety.
Brakes are critical in the snow. Disc brakes are the best, U or V brakes are fine, but whatever brakes your bike has, keep them well maintained. Also pay close attention to the chain, keep it and the wheel bearings well lubricated. Local Twin Cities bike shops such as Paul's Bicycle Repair Shop will tune your bike up for winter riding. Arriving at your destination with your back covered in slush is never good, so fenders are a good accessory. Lower the seat a little from the summer setting for a lower center of gravity. You want control, rather than speed in the winter.
Clean Your Bike Motorists clean their cars to keep salt and grit from corroding the bodywork, and bikes should be taken care of just the same. Wipe down your bike frequently to remove snow, ice and dirt, or use a bike wash such as the one at the Midtown Bike Center on the Midtown Greenway.
Winter biking takes a little more preparation than summer biking, but it can be done, and it's a good way to get exercise and fresh air, and perhaps even be fun, in the winter months.