Make sure that the condensation pan is not overflowing, and causing water to collect in your attic insulation or floor. Mold will grow in warm, moist conditions. The water that collects in the pan should be draining out. If it isn't, the drainage line may need to be replaced.
Regularly clean all of the air conditioning filters in your home, and allow to dry before reinstalling. The idea is to remove as many of the microorganisms as possible, without adding water or moisture to the system.
Have your HVAC system and duct work regularly maintained by professionals. Semi annual checkups are recommended by many air conditioning companies, and some offer maintenance and prevention policies. Prior to the AC season, a technician will perform regular maintenance on your system, identify any problems and recommend repairs. Again, before it is time to turn on your heater in the winter, he will return and give the heating system a thorough checkup.
Clean the air moving through your system with ultraviolet light, which acts as a mold air purifier. A UV light, which should be installed by professionals, is mounted in the duct near the condensation pan. The purpose is to prevent mold growth. While UV lights will not sterilize all of the air moving through the system, it will help keep mold from continuing to grow. A well designed UV light system should produce as little ozone (another irritant for people with respiratory problems) as possible. UV light bulbs will need to be replaced periodically, and can be done by a technician during your scheduled maintenance.