As a St Louis and St Charles MO home inspector, I most often find central 'split' air conditioning systems. There is some maintenance to these systems to keep them working to their highest efficiency, and from failing prematurely. Inspect these items inside and outside the home regularly.
* Turn off the system before doing any of these suggested items.
The basic principle in these, is that, the air in the home is blowing through a cooled radiator, or 'A-coil', in the central system. Generally, but not always placed directly above the furnace, and usually not visible without removing portions of duct work, this A-coil is being cooled by changing the pressure levels of the liquids/gasses which are flowing in a loop from the condenser outside into the home, and then back out.
The same, but opposite basic actions are happening outside, as the radiator there, within the 'condenser' system, heats the air being blown through it. Said another way, it is cooling the gas in the line enough to become liquid again, and then pressurizes that liquid and sends it back inside, to be released into the A-coil, through a spray nozzle, converting it back to gas, which drastically reduces it's temperature.
That's about as basic as I can make it.
The main thing to note here, for the homeowner, is that there is a lot of air blowing through a lot of small fins and other areas prone to grabbing particulate from that air, and clogging the flow, both inside and out. This, of course, reduces efficiency, and overworks the system.
Keeping a clean filter in the interior portion of the system mostly guards that end, but, if it has ever been run without a filter, or, if the filter was so dirty that the air was forced around it, then the A-coil is likely dirty, and should be cleaned. They get dirty eventually, even with a regularly replaced filter. Usually a qualified technician is needed.
Outside, in the condenser, these (very similar) fins are generally visible, and can easily become clogged with grass clippings, leaves, cottonwood seeds, even heavy pollen, and, of course trash. You can clean these yourself with a garden hose, spraying with a low pressure from the inside out, through the fins, as needed, and, have a quality technician remove the cover, and really get it cleaned well, annually.
Bending the fins is easily done with just a bump, or even a bad basketball bounce. This is also why spraying with high pressure is not advised. 'Combs' are available to try to straighten out damaged fins, or a quality tech and do this for you.
Another thing to note while inspecting these items is the insulation on the line running from inside to out. Not only can poorly insulated lines reduce efficiency, but, since it goes into the home, it can also cause moisture damage to surrounding items, and even mold, from the condensation dripping off of it.
I've inspected well maintained systems lasting well past their normal life cycles, and, of course, poorly maintained systems that have failed prematurely. Keeping these clean, and regular professional maintenance, can get you the most for your air conditioning dollar.