Winter riding is fantastic -- far less sweat (if any) and fewer people clogging up the trails .

My #1 rule: If you break a sweat you already screwed up. Stopping to take off layers is a sign that you wore too much, not a planned option for me. I usually figure out what I need to start off cold, but end comfortable and not sweaty.

The best thing about this approach is that you can wear anything - I strongly disagree with other cyclists on this topic, but I wear almost nothing but cotton: t-shirts, LS shirts, flannel, twill pants, basically everything but socks (wool is better). I can't stand the feel and smell of poly-crap. And it doesn't breathe worth a darn compared to cotton. Yeah, it wicks sweat, but I have a closet full of polypro, polyester, silver-impregnated poly-stuff that I never wear.

Merino wool is great, but expensive. Sometimes it makes me itch still, but I have terrible skin. YMMV.

Winters in DC are mild, and have run at about 19F as a lowest in the morning, with 20-30's more common.