Having a high efficiency air conditioning system is where it starts. As of January of 2006, the minimum efficiency standard is 13-seer. That's up from 10-seer where it has been since around 1991. The efficiency rating is a number akin to the gas mileage of your car - the higher the number, the better. The range (as of Jan. 2005) is from 13 to 19-seer. You may be surprised to learn how expensive a 19-seer system is but at the rate utility bills are increasing, a 19-seer system will pay for itself in a few years.
The other consideration for saving money on utility bills is to attempt to contain the heat in your home (or prevent heat from entering), and to look for ways to cut consumption. Here are some very good ideas:
Lighting can account for 20% of your bill. Use dimmers on all your incandescent lights and fluorescent light bulbs where a dimmer is not practical.
Hot water can account for 16 to 25% of your bill. Turn your water heater down to 115 degrees F. and use cold water to wash clothes as much as you can. Also, replace the shower heads with ones that are rated as "low-flow."
Ventilate your attic. Excessive heat in the attic penetrates your insulation.
Add insulation to your attic. It is recommended that if you have less than 12", increase that to 18.
Programmable thermostats can save money by turning off your system when you are not at home
Consider zoning your home with something similar to Honeywell's Zone Control. Heating and Cooling only the portion of the home you occupy will save $$$. Why condition the bedrooms during the day when you are only in the kitchen/den?
Caulk around window frames and doors
Install foam gaskets around light switch covers and plug covers on exterior walls.
Change your furnace filter each month.
Keep your shades closed on the South and West sides of your house in the summer.
Here are some links for more money saving ideas.