Riding in cold weather.

OK,
So I've been riding since August of this year and try to get as much riding as possible. It has been getting very cold here in New England. I've been riding in 50 degree weather no problem but this morning when I decided to take the bike to work, it was 30 degrees out. I decided to take the city roads instead of the highway but even through the city riding, I was very cold by the time I go to work (20 minutes). The most problematic for me is the knees getting very cold. It's been 5 hours since my ride and my knees are still cold.

How many of you ride in below freezing temps? What are your ideas on staying warm? Any thoughts or comments?