Set your thermostat higher. For every degree over 78 degrees you'll save 5% to 8% on cooling costs. Few people will notice the difference between 78 degrees and 80 degrees, but doing so can result in significant savings. When you leave home for more than an hour, set the thermostat to 85 degrees or more (the room will cool down in 15 minutes once you get back) for even greater savings.
Close off rarely used rooms. Close doors and air vents going into these rooms so you don't waste air conditioning cooling a room no one uses.
Install a programmable thermostat. A programmable thermostat allows you to preset temperatures for different times of the day, so you can leave temperatures higher when you're out and cooler when you're home (reverse during the winter). A programmable thermostat is relatively inexpensive ($30 to $50), easy to install, easy to use, and can save you 10% to 20% on your heating and cooling costs.
Use a fan. By circulating the air a fan can make a room feel 4 to 6 degrees cooler. On very hot days a fan can supplement air conditioning and allow you to raise your thermostat a few degrees. On milder days a fan may be all you need to keep cool. They don't actually change the temperature though, so turn them on only when you're in the room.
Open and close your windows wisely. When temperatures drop in the evening open windows to let cool air in (you can help things along with window fans). Then, once the sun is up and temperatures start to rise, close windows and shades. Also, use kitchen and bathroom ventilating fans carefully when the air-conditioner is running. Use them only for the amount of time truly necessary or risk blowing your nice, cooled air right out into the neighborhood.
Use window treatments to block the sun. Keeping shades drawn or blinds down in rooms that get direct sun can cut down on the amount of heat entering your home during the day.
Consider exterior sun blockers. Awnings and shade trees are two other options for blocking the sun during the day. If you decide to plant trees, make sure to plant deciduous trees on the sunniest sides of your house. This way you'll get shade in the summer, yet can still use the sun's warmth in the autumn and winter.
Keep the air flowing. Make sure there is nothing, like draperies or furniture, blocking air vents. If you do find that your cool air is being wasted blowing up behind curtains, there are inexpensive plastic air directors you can pick up at the hardware store to direct the flow of air out into the room.
Use heat producing appliances during cooler hours. Try to avoid using the stove (especially the oven), dishwasher, and clothes dryer during the hottest part of the day since these appliances generate heat and can increase room temperatures. Now you have a reason to put off the laundry and barbeque!
Keep air filters clean. Check air filters once a month and clean or replace them when necessary. Doing so can save you 5% to 15% in heating and cooling costs.
Fix leaky ducts. Keep your air-conditioning running efficiently by checking for and repairing any leaky ducts. Leaks are most likely to occur near the return plenum, where branch ducts meet the trunk line, and where ducts attach to outlets. Also, make sure air ducts are properly insulated, especially those that pass through the attic or any other unconditioned areas. Save any major duct repairs for a HVAC professional.
Seal air leaks. Check the seals around doors, windows, and anything else that passes through a ceiling or wall (pipes, electrical conduits, kitchen and bath vents, etc.) and use weather stripping or caulk to repair any leaks.
Have a happy, safe, and cool summer!