At least 3 layers: a thermal base layer, a thermal cycling shirt and a wind/rainproof outer shell. The highest quality gloves are essential because the wind chill can take temperatures well below -20c when you are moving fast and the same goes for the feet – you need good quality overshoes. The pain you feel when frozen fingers and toes start to heat back up to their normal temperature is excruciating. If your ears aren’t covered then the same thing will happen so I thoroughly recommend a balaclava under the helmet.
In midwinter it gets light quite late in the morning and gets dark early. I try to avoid the biggest roads and so often ride in complete darkness and a good set of lights is therefore very important. The rear light is more important than the front one because it is the car coming from behind that is most likely to hit you if the driver can’t see you. Rechargeable lights are excellent these days and my front light is so good that from a distance I am often mistaken for a car with only one headlight working.
It goes without saying that you need to stop your bike in any weather but when there is snow and ice you need to be very confident in the ability of your brakes to do their job. For the last 10 years I have been using hydraulic disc brakes on my commuting bike and they are so good that I no longer feel confident when using the traditional “rim brakes”.
In the summer I ride on “slick” tyres with no tread to reduce friction so that I can go faster. For the winter I have got tyres with spikes and suddenly I don’t have to worry about black ice patches, in fact they work better on ice than they do on tarmac. Best of all they make a really cool “purring” noise.