Flatten your sleeping surface as soon as you climb in (use your knees): once it freezes, you're done
Create a "Trough" to sleep in
If multiple people are in a tent, put sleeping pads close together (head to foot if you need to): stop air from creeping in
Put your extra gear around the outside to insulate
Take your boot liners into your sleeping bag
eat a snack before bed when winter camping
Be prepared for frost: sweep it away