Insulate the crawl space
If your air handler is mounted in crawlspace or any other unconditioned spaces of the house then insulate those areas properly. Sealing will prevent the humid air from outside to get into the crawl space. It will eliminate or at least bring down the sweating problem in the exposed parts of the air handler.
Buy a Dehumidifier
If you are living in the southern states of the US, having a humidifier in the house is a pre requisite. Install one in the crawl space as well. It will maintain the correct humidity level in the house which will prevent sweating.
Change you filters at regular intervals
Dirty filters are one of the major causes of air handler sweating. The airflow is reduced due to blockage in filter. Consequently, the temperature of the air that reaches the evaporator coil goes down. The air being cool will not be able to give out enough heat to vaporize the refrigerant. The temperature of the coil will fall and it could freeze.
Leaky ducts
It is one of the major causes of sweating. Moisture and dirt trickle inside the HVAC system through the leaks. Repair leaks whenever you detect them.
Blockage of the drain pipe
If the drain pipe of the air handler is not working properly due to blockage or any other reason, it will cause sweating.