Preparing Your Bike
Bike: Be good to your bike and try to limit the amount of exposure to the elements it receives. Don't leave your bike outside all winter in the harsh cold and rain. If you take the time to wipe down your bike and store it safely indoors it will serve your needs for many seasons.
Lights: Stay visible! Rain and dark clouds make it harder for drivers to see bikers. Make sure that you have front and rear, bright well-functioning lights and change the batteries often.
Chain: Your chain will pick up lots of grime and debris from the road. Wiping your chain off with a clean rag when you get home from your commute will preserve the integrity of your chain and keep your bike shifting smoothly all winter. After wiping the mud off your chain, you need to apply a layer of chain oil to it. There are wet/muddy specific lubes that work best.
Tires: Are your tires beginning to go bald or looking rundown? Chances are months of spring/summer riding have worn them down. It's a good idea to have your tires checked by a bike mechanic and changed if necessary. Good tires will keep your stopping distance and cornering abilities safe.
Fenders: A good set of fenders can be the difference between a winter commute full of misery and one of ease. A FULL set of fenders with mudflaps, not clip-on fenders, will keep you happiest by keeping you and your bike drivetrain cleaner; making wiping your bike down and oiling the chain simpler.
Brake Pads: Stopping distances increase in wet conditions. Changing your brake pads when the rains come will keep you stopping confidently all winter.