Install a programmable thermostat in your home. Programmable thermostats allow you to precisely set the temperature throughout your home.
Set your thermostat to "Auto" so that the air conditioner or heater only runs when needed rather than running continuously.
Set the thermostat during summer months so that it doesn't cool your house at times you aren't home and does cool it when you are home. For example, if you work during the day, you can set your thermostat to about 90 degrees in the summer during the workday and 78 degrees while you're home.
Set the thermostat during winter months so that it keeps your house at a lower temperature when you are not home than when you are home. Using the same example as above, you might set your thermostat to 55 degrees while you are at work and 68 degrees while you're home.