A Buff is indeed well worth packing, not least because it fits neatly under a helmet. Call me a pansy but I always think there's more chance of a head injury in winter.
On second thoughts, don't call me a pansy - I'm new here
Depending on your route you might find a pair of
Yaktrax extremely useful. My commute involves a number of quiet residential roads and sections through a rather open riverside park, all very prone to heavy ice. After a couple of bad spills I took to packing the Yaktrax, slipping them on and pushing the bike across the icy stretches if necessary. Probably not essential if you are able to stick to busier roads, though. More generally useful: I make sure to carry a down gilet or similar extra top. Stopping to fix a puncture is when you really start to freeze.