Mildew is one of my biggest hot buttons, living in the South. HATE IT. ugh.
Obviously, ventilating is the main key, but even that won't cut it here. One has to go on the offensive...!
3M has a great mildew remover product. I wouldn't use it on cloth or wood, but it's great on fiberglass, vinyl, and I just used it on our deck furniture and umbrella with great results. They also have a product to help prevent mildew, after you've gotten rid of it. On wood (where it does grow down here in the humidity!), you can use a bleach or vinegar solution, or even the 3M product if you get it off in the time it says. A stiff brush and mild bleach solution are probably best, though.
For upholstery, if your cushions are in OK shape (after cleaning), you can get a moisture barrier put on the foam (what they do for outdoor cushions for your home). You can also use Sunbrella fabric to slipcover, which can be thrown in the wash with bleach a hundred times (even the colored stuff) and come out looking great. I had 2 couches covered Sunbrella, one white, one white with green stripes, when my kids were little and dogs were shedding, and they looked great way longer than the couches held up! We will be redoing all the cushion upholstery in our Newport 33 this fall, and I will be using 100% Sunbrella with wrapped foam inserts.