About all you can do is to run the a/c unit for a few minutes every time you drive. This will prevent the front compressor seals from becoming dry. Otherwise, just be sure that the system maintains a full charge. If the refrigerant leaks out, moisture will work its way in, and that is bad for the compressor and expansion valve. But when the recharge day comes, you'll learn that there is no R-12 anymore, and you will have to convert the system to R134. There is nothing else that can be done with the refrigerant loop part. Do not know how the auto part functions on your car; if it is vacuum operated you can only see that the hoses are good; if electric motors drive the doors, then again there is not much that you need to do.