In Japan and other asian countries with wet seasons, fungus and other mold growths are a serious problem for camera collectors and users. How do you keep the little critters from growing on your prized possessions? Fungus requires two things to grow: moisture and dark. Take either of these away and you'll be spared the shock of finding your prize Summilux with white spiderwebs or spots growing on the lens surfaces.
Use it or lose it: Simply using your camera frequently is a good way to prevent mold from growing. Sunlight and UV light both kill fungi. Take your camera out often and the sunlight will naturally kill off anything in it. Nothing hurts a camera more than storage. Even if you can't use it, take the lens off the camera and put both in the sun for a few hours. It's important to take the lens off because the sun coming through the lens can burn a hole in your camera's shutter. Remember to take all your lenses, auxiliary viewfinders, and filters out for this treatment. Fungus is contagious.
Keeping it dry at home: In all of the asian countries I've been in that have a wet season, they sell dry storage cabinets for valuables. These can range from cheap oversize plastic tupperware-type boxes (~US$20) to fancy glass storage cabinets (~US$300). Most have built in hygrometers (humidity meters) and some way to control the humidity, such as rechargeable dessicant packs or electronic humidity controls. If you're staying in the country for a while, it's worth the investment. I bought one of the smaller plastic boxes for my stay in Japan.
In Japan, cans of nori (seaweed) come with giant dessicant packs to keep them crispy. I use those. It's easy to recharge them by putting them in the oven toaster on "low" for about 30 minutes. I've also sauteed them on a frypan as well as microwaved them. Anything that can apply a gentle heat of more than 100 degrees centigrade (212 farenheit) for more than 15 minutes works. If it's in a plastic wrapper, be careful not to melt the plastic, though.
In Japan, Fuji Color sells small satchets of fungicide (labelled フジカラー カビ防止剤 in Japanese). I haven't seen this sold in the USA. It contains a chemical called BCA which prevents fungus growth and is very cheap insurance (150-300). I keep one in the same dry box as my cameras and one in my camera bag.
Keeping it dry in the field: If you're travelling and can't lug around a giant plastic box, not all is lost. The most important thing is to stock up on dessicants. You'll need more because you won't be able to recharge them as easily. In Japan, as I said, each can of Nori comes with a huge satchet. But in the States, I buy mine in industrial sizes here: www.desiccare.com.
Dessicare sells industrial grade sizes as well as smaller satchets. Last year, I bought two of their huge shipping container dessicant bags, for use in my storage unit in Saint Paul; as well as smaller packets for my camera bags and indicator strips. They were very quick in shipping. They're a good company that mainly sells to other companies, like shipping companies and electronic manufacturers.
In the field, I keep several of the small packets in my camera bag. That keeps the general humidity in the bag down. I recharge them every now and then. If I don't have access to an oven or microwave, sitting them in the hot sun or in front of a hot incandescent bulb will dry them (slowly). The worst thing you can do is to keep your cameras + lenses in a camera bag that's damp or wet overnight. If your bag gets wet, dry it immediately. Hanging it in the sunlight will help kill any bugs in it.
The problem I find is that my day bag gets damp during the day if the humidity is high. So at night, I hang the bag to dry and take all the cameras and lenses out and put them in my hard-sided suitcase with a large satchet of dessicants. Hard sided suitcases are relatively air tight. Take all your clothes out when you do this because clothes have a high level of residual moisture.
Remember to recharge your dessicants. They're worse than useless if saturated -- they'll actually elevate the moisture level.