I have not done any maintenance on my central air conditioning system since I bought my house five years ago. Everything seems to be running fine but is there some sort of maintenance I should be doing regularly?
Every year or two I call out a professional company to check the coolant levels and check for any mechanical defects since I don't have all the gear to do it myself.
Aside from that I just try and keep the area free of debris and anything that could damage the unit. In my case I have a tree off to the side and several branches hang over the unit which I trim back, don't want a branch falling off and spearing through the unit.
Most local HVAC service companies offer an annual service plan where they come out once a year (twice a year for a heat pump), check over everything in the system (indoor and outdoor), clean off the coils, replace the air filters, etc. They will measure the voltage and resistance of various electrical components in the system, as well as measure the temperature of the coils to see if anything is out of whack.
If you have a service contract, there really isn't anything else you need to do for maintenance except make sure the outdoor unit isn't overgrown with grass/leaves/whatever.
Around here, a service contract for a heat pump (2 visits per year) runs $150-$200 per year. I think it's in the sub-$100 range for just an A/C unit, since they don't have to care about both heating and cooling.
I also have a humidifier plumbed into the HVAC system with a moisture pad that has to be replaced annually. Replacing the moisture pad is covered as part of the my service contract (as well as the air filter).
Most of the HVAC companies around here will also give you a small discount on repairs if you have a contract with them (call you a "Preferred Customer").