Well, what you can do is every time you're done using AC, turn it off and then run the fan on high for 3-5 minutes to dry out the condenser.
If the mildew smell gets too bad, you can use this Toyota AC refresh kit 00289-acrkt.
What it does is it shoots bacteria killing foam into the AC box killing all the fungus in it.
I would recommend doing it at least once every two years.
Do what jason says or another easy fix (if the mildew didn't spread to other parts) is to replace cabin air filter - just replacing the cabin air filter on my dd each year helps (it gets mildewy about a year from using ac & not properly drying out w/ fan on ac off for last couple miles of driving).
Aside from the toyota ac refresh kit, there are other kits available, too, such as Klima Cleaner. Then just use a odor eliminator "bomb" like Meguiars. These are typically used on older cars where the smell has been there a while & embedded.