Think layers and include a top windproof layer. I have a PI Optik jacket that I use as a top layer; it lets me ride in about 10 deg colder weather for whatever is underneath. Don't forget extremities - full-finger windproof gloves - and eye protection (even clear lenses) to keep wind off eyeballs. I wore out a pair of PI Amphibs shoe covers over a couple of seasons of winter riding and commuting and am shopping for their successor. Legs are easiest - they're already generating most of the heat anyway. Leg warmers can get me down to about freezing, I add a water/windproof layer (REI Novara rain pants in my case) for below freezing/wet conditions.