Keeping many valuable Collectibles dry is your first step to protecting them from various other dangers. Bugs -and other harmful living things- need water. Particular harmful chemical reactions are at the very least- aided on with moisture. Complicated subject and a lot of it involve too much chemistry. Perhaps the simplest and easiest advice I came across would be to put a pound or two of rice in the oven at about 300 degrees for one hour or two. You then place the rice in a shallow tray so that it can suck up moisture from the air. But in case you have read my post on bio-hazards -especially silver-fish, you are aware this is a terrible idea.
Another bad idea is to use calcium chloride. This desiccant pouch is the stuff they place on icy-roads in certain areas of the world. It soaks up moisture very well indeed, but as it does to, it turns to liquid itself -and no old liquid -it turns to acid. Joan Hammond -the firm chemist for the Drierite Company- sez, This could be trading one problem for a larger one. It is the stuff that destroys cars.
And this brings us The wise way to do it. You use a correct desiccant -gypsum -the exact same stuff they use to create sheet-rock. However, this is especially prepared gypsum. Among the things that they do with it is to put a chemical in it that changes from red to blue as it gets full of water. When it becomes full of water, you have got two choices -you either throw it or start over -and it is not really that expensive- or you also set it in the oven -like the rice- and cook it dry and use it over and over again. The water absorbing ability does not diminish, but the indicating chemical wears out. It does not really wear-out -is migrates in the granules of gypsum and has thin enough on the surface to the stage the color change does not show. Gypsum -by the way- is completely harmless -have a bowl of it for breakfast. However, the moisture indicating chemical is poisonous.
The Drierite Company will be happy to offer you the small Amount a collector or hobbyist might use. They will also offer you a little Aluminum container to keep it in. You can throw the aluminum and the desiccant It retains into the oven once the need arises. Then It is dependent upon your precise needs and the effort you are willing to spend. The Stuff is benign you could -for example -only spread some out in the bottom of your case or the back of a drawer. Have to sweep it up When it is done its job. Or you can set it in a small saucer of make a little Aluminum foil boat or even sew a small bag with cloth that is porous. For that Mater, Dreirite makes small bags with the stuff tucked away inside. Here Is what they say on the topic of how large a bag for how large a space.