Lot’s of good points already, I’d emphasise:
– Pick your spot, make sure the tent is sheltered (but safe!)
– Anyone you can share a tent with? More bodies = more heat.
– Decent down bag with a liner, close the hood. Make sure the bag stays dry – I tend to keep mine in a dry bag whilst it’s packed and I’m hiking.
– Decent insulating mat, and make sure it’s full length – I have a 3/4 length one for most uses (feet on bag if it gets cold) but when it’s very cold it’s not good enough.
– Make sure you’re well fed
– Keep water in the bottom of your bag, along with damp (but not wet) gloves etc.