I use kitty litter that has been coated with PG. I take fresh silica gel (crystal kitty litter) and add enough PG (100%, not the 50% "cigar juice") to wet them out evenly. I allow them to dry, and they are ready to go.
The PG acts as a humectant, slowing down the evaporation of water. The crystals won't hold quite as much water as untreated silica, but I find that they don't spike humidity either after being charged and stay wet longer.