A good thermarest pad, like a Prolite 4 is going to help a lot for starters. As for the bag, if you want to keep your existing 15 degree bag, you could add an overbag to stay comfortable at far lower temps. The FTRSS (Flexible Temperature Range Sleeping System) overbag that Wiggy's sells as part of the military 3-in-1 system is large enough to contain a normal mummy style bag without compressing the insulation. They are very durable, can get wet and still insulate, etc.

(Wiggy's makes a -60 degree bag!)
http://wiggys.com/category.cfm?Category=6