Layering is key: Wear moisture-wicking base layers, an insulating mid-layer, and a windproof, waterproof outer layer.
Keep your feet dry: Wear woolen socks, and carry extra pairs.
Use a good sleeping bag: Ensure it is rated for -10°C to -15°C or lower to stay warm at night.
Keep your extremities covered: Wear thermal gloves, hats, and balaclavas.