I do pretty much what Salsa_Lover said when winter cycling - just have a lightweight jacket on the outside, and then have layers underneath. I recently bought a super-bright neon yellow jacket that I use when riding around town. I think it should work year-round: In the winter with a shirt and sweater underneath, in the autumn and spring with just a shirt under it, and in the summer the arms can be detached and it becomes a light vest with a mesh back that I can throw over a shirt when it's dark out.

The hands and head are more difficult to get right in the winter. On your hands, you want some warm and windproof gloves, but not too bulky so that you can still operate the brakes and gears easily, which is why I just bought the extra liners for my other gloves instead of getting bigger, bulkier ones. For the head, I'll wear a cross-country skiing hat under my helmet when heading to work (6kms of downhill) but won't bother when coming home. If I'm not wearing a sweater that comes up around my neck then I'll put on a small triangular scarf thing like this to keep the breezes out, but is easy to remove if I get too warm. Heading down the hill I'll normally put on a pair of riding glasses with either clear or dark lenses to prevent me from crying in the wind, but again I don't normally worry about these when coming back up.

In Canada, where I used to commute in temps down to -20C, I would use a pair of clear ski goggles to protect even more of my face, and in those situations I would wear a winter jacket, but normally with the pit-zips open.

EDIT: BTW, there is an entire sub-forum devoted to winter cycling over at bikeforums.net, with a sticky thread entitled "Today, I Wore ...", you my find some of the hundreds of responses in there interesting reading.