Stuffing extra socks into summer-weight boots reduces healthy circulation making your feet colder and offering frostbite a chance to sink its teeth in. Middle layers are for warmth, and they should be adequately large. No tight-fitting clothing when insulation is your goal. Peel off layers as you warm up to prevent wetness from sweat, and add them back on as you cool down. To help maintain dryness, pace yourself.
Overexertion means over-sweating, and it burns up energy required for warmth.
Keep your head and neck well-insulated. Wearing a scarf ranks high as a healthy idea. Scarves can be pulled up over your mouth to pre-warm the cold air you're breathing. Wear mittens, not gloves, to keep your hands warm. Drink lots of water.