What type of regular maintenance should I be doing on my central air conditioning unit?
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.