Now is a good time to buy outdoor gear, because sales, and boring people don’t like being outside in January.
Put warm things on as soon as you sit down. Do not wait until you get cold to put warm things on - aim never to be cold in the first place, whether through movement or layers. If it is getting dark, or the weather is turning, or you are sitting still, put more clothes on now, not later once you’ve already lost heat.
Merino thermals, if you are okay with wool, are safer around fire. Synthetic fabrics are like wearing petrol, and will melt into burns, whereas natural fibres will just singe and fall off you.
Warm hat, scarf, gloves, socks - Sealskinz waterproof socks will keep your feet dry in most circumstances, though they can be a bit sweaty.
Try to make sure that every single thing you have brought with you can be worn at once if it’s really necessary. Hopefully you won’t need to, but it’s better to have seven layers on and be feeling like the Michelin Man than to be cold and miserable.
Warm lower half is important too. For some reason people seem to pile on jumpers, then just wear one pair of trousers. I’m expecting to be wearing at least fleece lined trousers (£15, good for monstering and running around in as they are) under my kit for everything. If it’s really freezing, I’d be wearing thermals, tracksuit bottoms, fleece lined trousers, waterproof trousers, kit trousers, kit thick leather skirt, all at once.
Warm bottoms are particularly important if you will be sat on the floor, for example in Conclave. In Guides we’d make camp sitting mats out of newspapers wrapped in plastic bags - to make these IC you could stick an old pillowcase around them. Bits of Karrimat or carpet tiles also work.
Look after your friends. If you see someone else is getting cold, do something about it. Do not let them sit curled up getting more and more cold and demoralised, because in a few hours you could have a more serious problem. If you can bring spare warm things for lending to other people, that’s good. If you can’t, see if you at least know of someone who can, so you can send cold people to them.