Hiring a Professional - depends on how old the unit is and when you last had a full service done. This is much different from just cleaning/replacing the filters but doing the latter will make your aircon unit work better and prolong it's life.
"When your air conditioner needs more than regular maintenance, hire a professional service technician. A well-trained technician will find and fix problems in your air conditioning system.
The technician should:
•Check for correct amount of refrigerant
•Test for refrigerant leaks using a leak detector
•Capture any refrigerant that must be evacuated from the system, instead of illegally releasing it to the atmosphere
•Check for and seal duct leakage in central systems
•Measure airflow through the evaporator coil
•Verify the correct electric control sequence and make sure that the heating system and cooling system cannot operate simultaneously
•Inspect electric terminals, clean and tighten connections, and apply a non-conductive coating if necessary
•Oil motors and check belts for tightness and wear
•Check the accuracy of the thermostat."