As someone who lives where the average winter temperature is -17C (with annual dips down to the -40'sC, I second the hat comment: wool is best, but synthetic fleece is a reasonable sub. Also, long underwear or the bottom half of a fleece jogging set worn under trousers is very helpful. Sitting around in the cold is bad, so you need to insulate as much as is comfortable for the work you do. Mitt or gloves are a good idea too. If you have to take them off for fiddly stuff, fine, just wear them the rest of the time. And stay warm...!!!