Make sure your attic has proper ventilation to exhaust trapped heat and keep things cool overhead. Consider installing a thermostat-controlled attic vent fan. Check to see that the room temperature drops no lower than 78 F for comfort and low energy usage. Turn your air conditioner off if you are to be out of the room for a substantial part of the day. Keep furniture and other objects placed so that they don't interfere with the free flow of air into and out of your air conditioner.
Some models are more energy efficient than others.
The higher the EER number, the more efficient the unit and the greater the money saved. Compare the EER for different models in the size you need. Do not base your purchase on price alone. Choose an air conditioner based on your needs. If it's too large, it will cycle on and off too frequently, decreasing the efficiency of the unit in cooling and dehumidification.