This is an older article, but it still applies today…
Quoted from http://moneypit.com/print/3338:
Air Conditioning Efficiency: How to Lower Home Cooling Costs
By Tom Kraeutler
Created 07/27/2009 – 13:02
Efficient air conditioning is the result of year-round efforts, from pre-season maintenance of your system to smart improvements inside and outside of your home. Take time now to consider the following air conditioning efficiency checklist so that your family is comfortable and your utility bills are manageable through the warmest days of the year.
Air Conditioning Equipment
- Schedule an annual air conditioning system tune-up: Annual maintenance of your air conditioning equipment  is a must for efficient, effective cooling during warm weather, and also contributes to the longevity of the components. As soon as outdoor temperatures reach 60 degrees, you’re good to book your HVAC contractor for a pre-season call including such checks and adjustments as cleaning the evaporator and condenser air conditioning coils, addressing refrigerant levels and cleaning and adjusting blower components.
- Shade the air conditioning compressor: Ample shade for your system’s outdoor compressor will help it to work more efficiently. Just make sure that there’s plenty of space around the compressor for proper air intake and output, keeping shrubs and other plantings a one-foot distance away from all sides of the unit.
- Seal air conditioning ductwork: Up to 20 percent of the cool air your system generates can leak out through poorly sealed and insulated ductwork, so take time now to eliminate those escape routes. Use duct sealant (mastic) or metal-backed tape (never duct tape, as it doesn’t have the required staying power) to seal all seams and connections, and then follow with an insulation wrap.
- Install a programmable thermostat: Programming air conditioning according to your away-from-home schedules yields significant savings while keeping you and your family comfortable during at-home hours.
- Change HVAC filters regularly: Check and change filters once a month or more during AC season. A dirty filter makes your system work harder by slowing down air flow, and also wastes energy and threatens components with dirt and dust buildup.
- Know when to upgrade: If your air conditioner is over 10 years old, it may be due for replacement with an Energy Star qualified model . As you shop, remember that new equipment needs to be compatible with the rest of your HVAC suite in order to be efficient, and other components may also require replacement if they’re of a certain age.
- Choose and install the right room air conditioner: The best and most efficient room air conditioning  comes from a unit that’s perfectly sized to the space and properly installed. Energy Star once again comes to the rescue with a collection of qualified room air conditioners  that use 10 percent less energy than conventional models and can be selected with help from Energy Star’s room AC sizing guidelines . For added energy efficiency, install your unit on the shadiest side of the house and select a model with easy-to-clean filters and controls such as a digital readout for the thermostat setting and a built-in timer.
Around the house
Here are several other ways to improve your home’s air conditioning efficiency that you can do around your house.
- Upgrade insulation: Keep that precious cool air indoors by adding insulation throughout your home. Some of the leakiest, most under-insulated homes are located in southern climes where winters tend to be mild, so bulking up insulation before high heat arrives is an absolute must.
- Keep storm windows closed: The same air that leaks in during cold weather leaks in during summer and drives up cooling costs, so keep storm windows closed in rooms with window air conditioning units for extra cooling comfort and efficiency.
- Pull the shades: Keep rooms cool by closing shades and blinds during sun-filled hours of the day.
- Spin out coolness: If you have ceiling fans in your home, get them spinning in the right direction for cooling efficiency by setting their reversible motors for counterclockwise motion that pulls cold air up.
- Landscape for shade: Strategically planted shade trees not only ad interest to your landscaping but also keep your home cool during the summer months, trimming your air conditioning expenses by as much as 25 percent.
- Work at night: Keep your home cooling efforts efficient by limiting use of heat-generating appliances like clothes dryers and dishwashers to evening hours.
Improving your home’s air conditioning and cooling system efficiency  can be achieved through a combination of common sense maintenance tips as well as by reducing the heat that gets into your home. Follow these steps and you’ll improve your air conditioning  efficiency, reduce cooling costs and improve your comfort all year long.