Plenty of thin layers are the way to go.. Bit embarasing this, but i actually use thermal tights & socks. then leggings ( aldi bike gear type) under my leather trousers, and a couple of layers of thin long sleeved t shirts & sweat top under my leather jacket. I very rarely get cold.. But if you do a top tip is to stop and buy a newspaper and stuff that down the front of your jacket, that makes a hell of a difference and helps to keep your body core temp ( and, your jingly lady bits) warm. Heated grips do make a big difference too.