Install an attic fan to help ventilate some of the hot air out of the attic. Attics can reach temperatures of 140 - 150 degrees and this build of warm air can make the upstairs of a home warmer than it needs to be. This makes the air conditioner work harder to do it's job. It costs you more to cool your home and the life of the system can be decreased.
Replace older windows with more efficient low-E glass. These windows help to reduce the effect of the suns warming rays. Also, keep storm windows closed. The above 3 tips will reduce the heat load gain on your home which means your air conditioner doesn't have to work as hard to keep the home at a comfortable temperature. This saves money and extends the life of the system.