In short you can't. It's a problem with any cold surface/warm air junction. Allowing a lot of ventilation may help a bit but that means having a gale blowing through underneath and you may not want that. Or avoid putting 'warm' things in there such as cars just in from a run or tumble dryers. Let a car cool down for half an hour or so before you put it away, especially if it is wet from rain as well.
Andrew Smith
December 2005