For the base layer on your upper and lower body, wear a lightweight material that wicks away perspiration quickly. Marino wool is preferable because it retains body heat and does not build up smell from perspiration.
Insulating layers on your upper body should zip so you can ventilate when you are hot rather than trap in sweat. In extreme cold, you don't need to worry about rain, so a down coat for your outer layer is best. A high fill count (550 or 700) will keep you warm. It should also be windproof. On your lower body you'll want Gore-Tex pants that are windproof.
Head and Neck
Keep your head and neck covered. A balaclava covers your head, face and neck with an opening for your eyes. If you get hot you can pull it down to your neck. A balaclava under a down coat with hood is ideal.
Mittens always keep your hands warmer than gloves with fingers. Ideally, your mittens should cover your wrist and have a waterproof inner and outer layer. The inner mitten should have an index finger for more dexterity.
We used to wear two pairs of socks, mostly because the wool ones were itchy. Today, you can get thick, non-itchy mountaineering socks so you need only one pair on your feet. Over that find a boot that covers your ankles: either a waterproof, seamless boot like a Sorel or one that has two layers, leather on the outside and a waterproof Gore-Tex inner shell.