Planning: Winter, with its short days and questionable road conditions, is no time to experiment with new routes, at least during the early morning and early evening hours of your regular commute. Stick to the routes you know; if you want to develop some new routes, do it over the weekend, during the day.
Clothing / Accessories / Equipment: I like a layer of full-on fleece (say Polartec 200 gauge) next to my skin, topped with a wind-proof (but uninsulated) shell, and full tights. For mid-20 to 30 deg F. weather, this is really enough clothing. If it gets down into the zero degree range, I might add another layer, but it's possible to sweat this up pretty thoroughly in sub-freezing weather. I find that I warm up about 1.5 miles into my ride, where I hit my first significant uphill. Truth is, where you're exercising, you adapt pretty well thermally.
Protection of Extremities: I have a thin fleece cap that fits under my helmet (I'm probably going to add a balaclava), and I use SmartWool hiking socks. So far I haven't found the need to go to a second layer of socks -- I probably would use a vapor barrier wrap made from bread sacks or newspaper bags before I went to a full-on multilayer sock approach. (For a good discussion of why you want vapor barriers at your feet, go to this link at the venerable Stephenson "Warmlite" site.) The big difficulty I find is keeping my hands sufficiently warm. I wear a light Polartec glove liner under full winter cycling gloves, and my fingers still hurt from the cold at the end of a 9-mile ride. Better to hurt than to have lost feeling, I guess. The next step is to go to "lobster gloves" -- I've ordered some from LL Bean (Pearl Izumi brand) and maybe can find some comfort.
Other Equipment: One thing you'll need is sunglasses for the morning commute. The sun is low in the sky, and creates a lot of glare. If you don't wear glasses anyway, you'll need eye protection for the evening commute, also. One other thing that I've found, is that it's extremely handy to have a visor on your helmet. This is so you can put your head down to deflect the headlight beams of oncoming cars on dark roads. This keeps you from getting blinded.
Outfitting the bike: Mudguards (fenders) are essential, of course. Lights that meet the legal minimums also. I like the blinking-LED variety, for the way they conserve batteries. I've also added a large amber auto-reflector to supplement the standard red rear light. If I know the weather is going to be inclement, I don't plan to ride. But I carry extra clothing (a full rain outfit and the aforementioned newspaper bags) to keep me reasonably dry if the weather turns foul. As for tires, you can research and buy studded tires at Peter White's excellent website. For me, right now, if the weather is such that I'd need studded tires so I can ride on ice, hmm. I think I'll not ride that day. That doesn't mean you can't, though.
Motivation: Many of the other blog articles on winter cycling talk about motivation and getting going. It is true that the first mile is the hardest. I think the strategy that has been most successful for me is to just not quit, not get out of the habit. I do think that deep winter would be a challenging time to start a cycle-commuting habit, to say the least. I feel like any week where I can ride 3 days or more is a good week, and I've had mostly good weeks since the end of summer.
If you're planning on regular winter cycling, I hope these little tips have been useful. Be safe!