You need to prevent the buildup of mildew in the system during the colder months, as that's what causes the smell.
One trick I've heard is to make sure you run the A/C regularly (at least once every couple of weeks) during the cooler period, so that any spores get blown out of the ducts before they get a chance to settle and grow - a useful side effect is that this also keeps the seals lubricated and so increases their lifespan.
Have a look at http://honestjohn.co.uk/faq/air-conditioning