I agree with Romania's suggestions of a sleeping pad and the Thermo-Lite bivy sack. I have used a Thermarest pad for over ten years. It has kept me warm and comfortable on some rough and cold ground. I haven't used the bivy sack as an overbag, but the theory speaks fot itself.

If you want to avoid the expense of buying a new bag, you could add a fleece liner to your existing bag. Wearing polypro or wool long underwear inside the bag will also help. Finally, try wearing a fleece or wool hat when you sleep. My wife does this and swears by it.

Good luck,

Steve