As the external temperature increases your air conditioner has to work harder and more often to keep your house cool. You can help reduce this problem by insuring your house is properly insulated and that you are not losing cool air through cracks and holes, mostly around windows and doors.
Make sure your air conditioner has been cleaned recently. Blow out the filters and condenser coils. If it still doesn't work as well, be sure to have it serviced and the refrigerant topped up.