There are desiccants available that B&H and elsewhere that come in tiny tubes, and they'll turn pink once they're saturated. I have a bag of them and throw one in my bag when I travel for 1-2 weeks at a time. I figure it might help a little, but they're always pink by the end of the 2 weeks, usually much much sooner especially if it's damp or raining where I go.

Otherwise, everything of mine goes into a drybox at home, bags and all, with a desiccant inside. I got a clear storage tub from the Container Store that is meant for damp basements, I put a large desiccant inside, and measure it with a $10 digital humidor gauge.