I ride all winter if the temp is 15 F or above and if there isn't too much salty slush on the road. I have found a couple of ways to keep warm that you might not have considered before. I ride a fixed gear Surly Steamroller. Riding fixed means my legs are always pumping and I have found that this helps keep the blood circulating. I sometimes flip the wheel over and ride with a single-speed freewheel, so I can attest to the difference. Another great thing about riding sindgle-speed is the simplicity. I commute 4,000 miles a year, and in ten years I've had one broken chain and a handful of flats, that's it.
Because I ride a single-speed, I never have to shift, so I can wear mittens. I found a pair of snow machine mittens that are super wind resistant and, when I'm on a flat stretch I curl my fingers into a fist to warm up the tips.
A third strategy I've discovered is to take an old pair of heavy boot sox and cut the feet off of them, leaving just the tubular uppers. I pull these up over my knees, cuff down and then pull my tights on over them. This keeps my knees toasty, avoiding that brittle achey feeling they can get in the cold. If you keep the cuffs down, they won't curl up as you slide the tights on.
Hope this helps some of you to venture out a few more days this winter.