Glenn has asked all the right questions. Having a good pad is essential to keeping warm in cold weather. On snow, I use two-one closed cell foam and one Therm-a-Rest stacked.

Stuffing clothes in your bag may be counterproductive. See that parka I am wearing? If I have it with me, I may toss it over my bag, but not try to stuff it in the bag. Works pretty well. FYI, that parka is a TNF down parka and weighs about as much as a 20F bag.

Two suggestions - wear a base layer such as Capilene (Patagonia) or Merino wool. I have Capilene and have owned the same set for years. It is pricey but very warm and durable. Some people carry a separate set just for sleeping. I just have the one, plus I now have an expedition weight top as a third piece. A fleece or wool beanie, light gloves and socks might help. I've tried all three, but they tend to bother me, so I usually wind up taking them off.

Eat something before turning in. I suggest an energy bar of some sort - Clif Bar, Luna, etc. whatever you like. If you wake up cold, eat half a bar. You'd be surprised how well that works, at least it does for me and I'm very thin.