Install programmable thermostats. In summer, close window blinds during the day to keep out the heat. In winter, open blinds to let in the sun. Install shading screens on south, east and west-facing windows that are not shaded. Reflective film applied directly to the inside of the window reduces solar heat by reflecting it away from the window.
Awnings keep sun off windows in summer.
Installed at the proper angle, awnings allow sun to enter in winter when extra heat is welcome. For best results, plant two trees on the west side, and at least one tree on the southwest side, particularly in front of windows. Fix leaks and caulk cracks in walls, windows, doors, ceilings and floors. Even small holes can cause substantial cooling or heating losses. Install or repair weatherstripping and caulking to stop air leaks.