Try spraying some Lysol into your AC system about twice a year. It kills the mold that builds up during the winter when you normally wouldn't operate the AC. (do that for each of settings: 'fresh outside air', 'recirculated inside air', 'defrost', 'heat', etc.)

Also, it helps a great deal to run the AC for a few minutes approx. once a week for that will not only help reduce any stinky (and potentially unhealthy) mold from taking up residence in your AC system, but it also lubricates the seals in the AC system. If you don't do that, your AC system's seals will dry out quicker and that's an expensive fix. The refrigerant in the AC system contains an 'oil' that is the only source of lubricant that is in contact with the seals. . .