I agree with the previous posts, Attic first (assuming floor is insulated or slab), but also sealing around the exterior doors and windows. My old house had single pane windows with storm windows outside. In each room, I picked one window to open and the rest got caulked shut (might label the window as an escape). It saved a whole lot of money, furnace ran probably half the time it did prior. Keep in mind each person has to think of their own safety and how to escape in the event of an emergency, so other methods should be considered, like new windows, window seal kits, or plastic over the windows.