Your local heating contractor or WECC can tell you how. Open shades during the day to let sun in and pull shades at dusk. Don't use regular fireplaces to heat the house. Keep fireplace dampers closed when not in use. Move furniture blocking heating vents.
Check ducts for obvious air leaks.
Minor duct sealing can be done yourself. A professional contractor may be needed to tackle bigger jobs. Log onto our HouseWorks program webpages for a list of contractors specially trained to find hidden air leaks and diagnose other household problems. Clean or replace your furnace filter or air cleaner once a month or as often as necessary to keep your furnace in good working condition. Use bath and kitchen exhaust fans just as long as needed.