[onewhippedpuppy]

Most high efficiency wood stoves/fireplaces use a dedicated air inlet from the outside. I would think fabbing something similar (as suggested) near your heater would do the trick. Then follow up with weatherstripping/caulking windows, caulking any gaps/cracks/etc, replacing worn door thresholds and weatherstripping, outlet gaskets, etc. If you have access, adding insulation to your attic is cheap and easy. Even just going around the exterior of your home with caulking or expanding foam and sealing gaps can do wonders.