I don't really feel the cold all that much (t-shirt and shorts down to ~10 degrees) but here's my tips from camping in winter around home and living in the Bathurst area for a while (gets into the negatives). I'm sure most will have been mentioned but if I don't write all the ones I can think of I'll miss more of them.
Make sure your clothes fit snugly. The looser they are the more cool air can get around your body and circulate meaning your body heat won't warm it.
Wool, one light snugly fitting woolen jumper will keep you warmer than a big bulky polar fleece jumper. Also won't melt if you cop some sparks from the fire. Also saves you some space in packing. Wool socks, gloves, beanies and scarves are great as well. Woolen underlays, blankets and doonas for the bed will help keep you warm at night. Everyone buy wool everything
(son of a Merino stud owner). You can get underlays for chairs as well, I've found a jacket over the back of the chair works fairly well though.
Under clothing, such as singlets and whatever you call them pants things, tights? Even the normal cotton/poly stuff will be better than nothing, but the thermal gear would be best if you really feel the cold.
Wear your beanie and maybe socks and gloves to bed. The losing whatever % of heat through your head might be a myth, but the myth came about from trails that showed when in a nice warm sleeping bag with everything else covered you lost the most heat through your head. So if you are all snug with other gear on, you are losing most of your heat through your head.
Wind cheaters, make sure everyone has some wind proof clothing, a lot of the time the wind is the worst of it and wearing something to stop it is usually easier than trying to set up a wind break if there isn't anything handy.
Big Silver thermal backed tarp over the tent, added benefit of keeping it dry.
I've heard of people putting some hot coals into a thick glass jar and putting it in their bed to warm it, but never tried myself.