Other than the obvious, buying a warmer sleeping bag, what are some options a guy can do to use a 15-20* bag in colder temps? I can only pack in so many clothes. I have a BA Mystic SL 15. It's very possible I'll be using it come Nov in ND and it will be mighty cold. I think I read somewhere about guys using a bivy sac (??) inside of the bag to increase the warmth?

What are my options and what has personally worked for you?