If you're buying a room air conditioner, look for the ENERGY STAR® label.
ENERGY STAR qualified room air conditioners exceed minimum federal standards for energy consumption by at least 10%.

Buy the smallest air conditioner appropriate for the room you're cooling.
You'll be more comfortable and the unit will operate more effectively. See the chart below for the best size for your room.

Don't let the air conditioner bake in the sun.
Room air conditioners work best when kept out of direct sunlight. Install them near shade trees or on the north side of the house if possible.

Seal gaps along the sides of your air conditioner to keep the outside air from seeping in.

Caulk and weather strip your windows and doors, and seal any holes between the attic and your living space to keep cool air in and hot air out.

Clean air conditioner filters regularly, keep the front and back of air conditioners unobstructed.

Use fans on humid days to move air through your home. You'll feel cooler and you'll need less air conditioning.

Check your insulation.
Well-installed insulation is as important as the amount you use. Blown cellulose insulation is a good choice if you're adding insulation, because it fills gaps between fiberglass batts and conforms to framing irregularities.

In warmer weather, delay heat-producing tasks, such as dishwashing, baking, or doing laundry, until the cooler evening hours.

Use an exhaust fan to blow hot air out of your kitchen while cooking. The savings in your cooling costs far outweigh the electricity use of the fan.

Instead of an air conditioner, open windows on opposite sides of the house for cross-ventilation.

Use fans in your windows to draw in cooler night air. Close the windows and shades/curtains during the day to keep the cooler air in.

Use compact fluorescent light bulbs. Standard incandescent light bulbs lose 90% of their energy as heat. Compact fluorescent bulbs produce only a fraction of the heat and don't waste electricity.

Close window blinds, curtains, shades, or drapes in the daytime summer hours.
 Sunlight shining in windows usually adds the largest amount of unwanted summertime heat. During some of our hottest weather, it's even helpful to keep the windows themselves closed to keep hot air from blowing into your home. In the evening and early in the morning, open windows to allow cooler air in.

Plant shade trees to block heat from your windows and roof.