It was 25F when I left the house this morning - bitter cold for Oklahoma. I had a tailwind, but I wore an insulating jacket rather than my usual windbreaker. It was a bit too much. I was sweating by the time I arrived at work.
It's odd how some people react to winter riders. "How can you possibly DO that! It's SO COLD!" Then within a few minutes they're nattering on about an upcoming ski trip to Colorado. I'd much rather be riding my bike and staying warm from the exercise than shuffling in a lift line. To each his own.
There's a climber's adage that it's easier to stay warm than it is get warm. I'm a firm believer. I wear a lot of thin layers, all of them synthetics. If I get too warm, I can always peel something off. My pack has a balaclava, skullcap, doo-rag, and various gloves. My windbreaker sees almost constant use, with or without a sweater or vest. The one indispensable item is a good pair of tights. They're expensive, but the good ones are nearly windproof. My rule-of-thumb is to add a layer for every 10 degree temperature drop or if the ride is into a strong headwind.
The fixed gear helps me stay warm too. Sheldon Brown opined that "coasting is a pernicious habit!" It surely does make me cold on a long downhill. The fixie sees that I keep my muscles moving and stay warm.