Fans use less energy to run than A/C or your central air system. Before turning on your a/c or turning down your thermostat in the summer, see if using ceiling fans will help. Ceiling fans will hep circulate the air around you, which as it passes your skin will have a cooling effect. Set your ceiling fan to rotate counter-clockwise in the summer.
Also, don't shy away from box fans & circulating fans. These too will save you more money on energy costs that using your A/C. A well-placed box fan in a window in the evening can help draw in that cool air. Just be sure to remove the fan by mid-morning when it starts to warm up. That cool air you drew in from outside can keep your house cooled for quite a while, especially if you keep the shades drawn so that the sun isn't quickly heating up your house through your windows.
Fans used in your house in conjunction with you A/C unit can help you push air through the house so that the coolness reaches more areas, thus cutting down on the need for additional air conditioners. And if you have cooled your house down, turn off that A/C and let the fans push around the cool air to keep you feel comfortable.
In the winter, you can also use your ceiling fans. We all learned in grade school that hot air rises, so a problem in heating our home efficiently revolves around using that hot air that is being wasted at ceiling height. Set your ceiling fan to rotate clockwise during the winter months and it will push some of the hot air down to you.