The best time to service an air conditioner is before it's actually needed.
Regular maintenance and cleaning are extremely important.
Change the filter monthly. Other than the regular filter, there are additional ones that can be used, but check with an air-conditioning maintenance person first.
You can get a cap from a hardware store to put on the end of your air-conditioning drain. After capping it, find the end of the drain outside and use a dry vacuum to suck out the dirt. Be sure to remove the cap once it has been vacuumed.
An air-conditioning technician needs to use a high pressure vacuum to clear the drain. It should be done yearly to avoid any backup that could cause mold growth in your air conditioner and water damage to ceilings and walls.
To clean the air-conditioner coils, make sure the power from the unit is disconnected first. Wet the area you are cleaning with a garden hose. Using gloves and safety glasses, spray the commercial air-conditioner cleaner (available at an HVAC supplier) onto the coils. Let it set for a short while and then rinse it down thoroughly with the garden hose.