Don't touch it!
The only preventive maintenance I can think of is keeping the condenser and evaporator (if you can get at it) free of debris. Organic matter can get wet and cause poultice corrosion of the evaporator requiring expensive repair, especially oak tree leaves or pine needles - both of which are very acidic. The front condenser should also be free of debris by flushing with water.
Unless you have a drop in performance (vent air not as cold as it used to be), the a/c system is sealed for life and should not be "maintained". When refrigerant needs to be topped off, it should be done with a set of a/c manifold gauges attached to high and low pressure ports to monitor system condition. Even some accomplished DIYers are hesitant to touch an a/c system. Yes, it is possible to put too much refrigerant or oil in the system!
95 E320 Cabriolet, 131K