EJ has a good point. After you re-insulate the attic, that has to be the priority, check your bill and see what happens. It should reduces substantially. If it doesn't I would then check all of your doors and windows. It is a simple but time consuming job though - remove all the trim around the doors and windows. You will probably find that there is no insulation in the spaces where the windows/doors are shimmed. Fill this with injectable foam insulation, let cure and then re-install your trim. I would assume that your home met code at some point so probably you may have sufficient insulatiomn in the walls. 2"x4" stud walls would have R-12 and 2"x6" stud walls would have R-20. Some R-2000 homes have R-30 insulation. Blown insulation is not something that one would see in walls anymore. It used to be used but because of it's settling characteristics it was discontinued here way back in the early '70's.
As I redo the trim etc., on my windows and doors I am using foam insulation to seal the air gaps which I still have. I used fiberglass to chink those spaces but it didn't work so well. Newer products provide newer solutions to very old problems
Low E and Argon filled windows will lead to energy savings as well, and should be contemplated especially if your windows are more than 20 years old.