Gel packs, as mentioned - don't really help for long. They'll end up getting saturated in short order and recycling them (via oven) is just a pain.
Best solution - whenever you're home, at the hotel, etc. just open the bag and let it air out. A closed, dark, damp bag with no air flow is a sure fire way to promote fungus. So store them open as well.
One minute or so in a microwave recharges the Dri Box. Yes they get saturated fast in a cloth bag, but they work great in cases.