Air conditioners -- like autos -- will give you much better and much longer performance if you do a little simple maintenance. Don't constantly run the unit at very cool temperatures at very hot times -- at, say, 68° F when it's a humid, 100°+ F day. That's like running your car over 100 mph in the desert. It causes it to wear out faster.
As with all vehicles, change the filters frequently. Iinspect them at least once a month. If they are dirty, change them.
Make sure the condenser (the part outside your house) is level and at least 3 inches above the ground. Most units have a piston and cylinder that lubricate from the bottom up, and they will last longer if they are evenly lubricated. Usually, unless the condenser is sitting on the ground and it is only 1 or 2 inches off level, you can level it yourself. Gently lift the front or back edge of the unit and insert a shim to level it. Be very careful not to damage the refrigerant lines!
Trim foliage back from the condenser. Manufacturers recommend at least 2 feet clearance around and 5 feet above the unit for proper air circulation and efficient operation. If part of the unit is too near a fence, consider installing screening or vents to allow air circulation.
Listen for unusual fan or motor noises. Inspect the coil fins for damage and repair, if it's not too extensive. Most hardware stores have special combs or tools for straightening bent coil fins.
As with your car, you should have your air-conditioning system checked by a licensed, qualified technician at least yearly. Make sure the technician uses a high-pressure vacuum to clear the drain to avoid any backup that could cause organic growth in your air conditioner and water damage to ceilings and walls.
