Good advice from everyone. I dress in layers, make sure to windproof (helmet cover too) and sometimes I'll warm up on the trainer before I go out, if I'm feeling particularly resistant to the idea of getting going in the cold.

I do loathe stopping for a flat when it's below 30 F, though. Usually won't ride outdoors if it's under 15 F on a calm day or 30 on a windy day. Though one year I didn't have any good options so I rode 6 miles to work all winter, down to -10 F. Now that's too cold for me to ride. Snowshoe, ski, hike, I'm fine. But below zero on a bike is a unique kind of cold.

That said, y'all know this site?

http://users.rcn.com/icebike/

Hard. Core.