Commuting by bike in the winter is a heck of a lot easier than you would think. With the proper clothing, equipment, and attitude it can be done in a cinch.
When it comes to cycling in the cold, the most important thing to remember is to wear layers. I have ridden in temperatures all the way down to -4 degrees F and been completely fine with proper layering.
Top Half: I base my layers upon the relative temperature outside, but what works for me may not work for everyone, so I suggest experimenting with your own body in this case.
40-50 Degrees F: Normal wear, meaning a wife beater and a long sleeve tshirt, with a wind breaker on top.
32-39 Degrees F: Moisture wicking base layer(such as under armour cold gear, or wool shirt), followed by either a long sleeve tshirt or a lightweight long sleeve wool shirt. Wind breaker on top as before.
20-32 Degrees F: Moisture wicking base layer, wool sweater, wind breaker.
0-20 Degrees F: Moisture wicking base layer, jersey or another moisture wicking tshirt, heavy wool sweater, wind breaker.
Below 0 Degrees F: The lowest I have ridden in was -4, with wind chill of -21; I wore Moisture wicking base layer, light wool sweater, heavy wool sweater, wind breaker; I was completely warm on top and dry.
Bottom Half: Now your legs are a little trickier to pin point, they warm up quicker than anything else since you use them the most.
40-50 Degrees F: I just wear normal pants, usually jeans or windproof running pants.
25-39 Degrees F: Thermal lined wind/waterproof running pants(picked them up at Goodwill for 3 bucks).
15-25 Degrees F: Either thermal underwear(long johns) or moisture wicking base layer, either jeans or wind/water proof pants.
(-) 0-15 Degrees F: Thermal underwear, thermal lined wind/waterproof pants, windbreaker style pants.
Extremities: These are very important, since they will be the first parts of your body which can sustain frostbite.
40-50 Degrees F: Light gloves(if any), normal crew socks and running shoes, ear band(fleece ear band sold in sports stores for around 5-10 dollars)
25-40 Degrees F: Heavy half glove/half mittens(purchased at a Tractor Supply sotre for 6 dollars, they are half finger length gloves that have a mitten piece than can be pulled over your fingers when needed...perfect for most any weather.) Wool hiking socks(smartwool 10 dollars) running shoes, fleece balaclava for head.
10-25 Degrees F: Heavy glove/mittens; wool hiking socks, thick crew socks, leather boots; fleece balaclava--and depending upon the weather a helmet cover...I use a plastic shopping bag...but Nashbar makes a fancy one for 20 bucks.
Anything below 10 F: Light gloves, heavy gloves/mittens; wool hiking socks, heavy wool socks, hiking boots; fleece balaclava, fleece ear band, helmet cover.
As I said these are examples of what I wear, you may need to adjust things a little to get comfortable. The general rule of thumb is when you walk outside you should feel a little chilly, not cold, by the time you pedal through your first mile you'll be warmed up just fine. Always make sure to inspect your bike before leaving so you can potentially avoid stopping and being out in the cold for long periods of time. If you feel you have under dressed, don't be afraid to turn around and go get another layer--better safe than sorry; if you feel you have over dressed, pedal easier or remove a layer, but never your outer wind shell...it is the most important layer.
To be successful at commuting during the winter you will need a few extra things for the bike that you will not usually need in the spring-summer-early fall months.
Lights: In the late fall and through the winter most of us deal with it being dark when we leave for work, and dark when we ride home at night. Though lights aren't necessarily for the cyclist's benefit, they are actually important so drivers and other riders can see you going down the road. Therefore lights are very important for winter cycling. You should acquire at least a head light and a tail light. I have a P7 flashlight which is one of the brightest lights you can purchase, I bought mine off of DealXtreme.com for 90 dollars with a mount, two batteries, the light, and a charger. For my tail light I have a Cateye TD1100 it has 10 bright led lights that have 5 different flash sequences to catch drivers attention, for a while it was the brightest light of it's kind, now there are brighter, but this one is still sufficient. The general rule is: The more lights you have on the bike, the better off you are.
Fenders: Some riders will disagree with the importance of fenders for winter cycling, but I am a firm believer in not having water/mud/snow spray all over my back and especially not all over my face. The concept is pretty simple, it is more likely to be wet in the winter...therefore you will need fenders.
Snow Tires: Now in Central KY where I live we don't get a ton of snow, usually we get one or two significant snowfalls per year. Therefore I merely have a back up set of wheels for my commuter bike that has knobby, mountain bike tires on them. It has snowed once this year around 6 inches and I spent nearly 2 hours riding around and having fun in the snow without any problem with the back up wheels.
However in areas that get more snowfall than a few inches per year might consider investing in a set of studded tires. Studded tires are usually mountain bike style tires with sharp pieces of metal sticking out of the knobs all over the tire. These studs actually give you a better grip when turning or trying pedal in thick snow/ice.
Homemade Emergency Kit: I personally believe it is better to be safe than sorry. Now my work is only 1 mile away from my house at the shortest distance, though I frequently extend my commute to being 5 miles or more each way. Therefore I could technically get by without an emergency kit, but it is best to have one. This is not anything elaborate, merely a few additions to your seat/frame bag/ backpack/pannier. An emergency blanket--you can get them online or in any first aid kit you buy at a drug store. Cell Phone--sure most of us already carry a cell phone, but if you don't have one, it may be a good idea to get a cheap prepaid cell phone with 20 minutes or so for emergency calls. Food-Be it a powerbar, a piece of fruit, granola, trail mix, or whatever you wish that is along those lines--if something was to happen and you were stuck walking in the cold or you were injured the food would act as a source of energy, and can warm you from the inside. Finally 20 dollars--You never know when conditions could turn for the worse and you may need emergency cab fare, or even money to buy a new tube.
Getting in the right mindset is one of the most important things for winter commuting. If you constantly think "I don't want to do it," or "It is just too cold," then you will never be able to commit to riding in the cold. Once you convince yourself that it is actually an enjoyable experience to ride in the winter you will be much better off. Half the battle is the mental battle.
One thing I do is try to get excited about the ride in the morning, I love to ride anyway so I just remind myself of that fact and try to plan what will go on on the ride in, sometimes I switch up the route I take to add excitement, other times I'll try to get to work faster than I ever have before; basically I make it a game to ride to work, which helps me forget about the cold.
Once you finally get out and ride to work a few times you will see how much fun winter cycling can be, you'll be hooked. Check out more information at Icebike.org --they truly have all the answers you need.