Thnik about the bits that will get cold first, fingers and tootsies, and make sure that they are well insulated. Protect your neck since there is significant heat loss there. Keep your core (torso) warm. A heated vest works like a charm and is an inexpensive investment. Motorcycle specific outerwear works better than the other stuff.
Cold weather wooses like me prepare for every eventuality: heated grips, heated vest, motorcycle specific outerwear, full face helmet always, scarf, high (waterproof, laceless) boots, insulated gloves, longies etc etc. The coldest I've ever ridden in was -5c and I have no burning desire (sorry about the pun) to repeat it.