Keep hydrated – drink plenty of water – even if you’re not thirsty. Dry, winter air can dehydrate you quickly and water helps your body generate heat. Mixing your water with lemonade powder or Gatorade keeps it from freezing as quickly as plain ol’ water.
Dress in layers: first a liner layer of long underwear (silk is super comfy), then an insulating layer of fleece for warmth, and then a wind- and water-proof outer layer.
No cotton – when it gets wet from sweat or rain it makes you colder. Synthetic materials still insulate even when they’re wet.
Make sure you have a hat. You lose 60% of your body heat through your head. Sleep in your stocking cap or balaclava to hold in the heat.
Carry extra socks and gloves. When one pair gets wet, slip on another pair and dry the wet ones overnight inside your sleeping bag with you.
Listen to your body – cold fingers and cold toes need to be addressed. Cold toes can be caused by boots being laced too tight and cold fingers could be caused by tight glove liners. The restriction of blood flow could keep you from warming up.
Hot drinks around the campfire are good for keeping you both hydrated and warm. Limit alcohol intake as it thins your blood and makes it difficult for your body to maintain warmth.
Eat plenty of high protein and high carb snacks, especially before going to bed to fuel your body to generate needed heat during the night.