Just in time for our January outing, Backpacker magazine has a series of articles about cold weather camping. A good place to start is the winter camping checklist, found here:
This site is linked to other sites on winter camping, so you should be able to find out anything needed.
(Note of caution: Backpacker magazine is for serious gear-hounds, so don’t worry if you don’t plan to bring everything they describe–it’s just to give you some ideas!)
Mr K.