If your sleeping bag is not warm enough, the ultimate solution will be replacing it with a warmer bag. However, in the meantime, you can use a sleeping bag liner which adds to the sleeping bag's warmth and breathability. You can also wear a heavy underlayer to keep yourself warm before slipping into the bag. If you are camping as a couple and the sleeping bags you have can zip into a double bag, zipping them together will make both of you warmer.