You have the infamous "aroma de mildew" that plagues many owners of modern cars. The BMW people complain of this all the time, also. First, try to clean your system using the Frigi-Fresh or the Wurth spray, as the other letter recommended. Definitely and clean out leaves and pine needles and change the cabin air filter.

As for prevention, it's tough. The only thing I know you can do is be sure to turn OFF your AC a few miles before you reach your destination. Then run outside air through the system to dry off the coils. If you leave the AC on until you stop, bacteria and other repulsive organic matter grows on the wet coils. Change the air filter annually. Good luck!