The easiest way to increase humidity is having standing water and or moist substrate in the tank. I keep my substrate moist enough that I don't even need to mist to keep the humidity up to reasonable levels, but keep in mind I use a clay based substrate so it doesn't decompose... It sounds like you maintain your tank regularly so I'd say go ahead and dump some water in your substrate, and use a slightly deeper layer that will hold more water. Also is look into leaf litter for covering the substrate, it will look more natural and help keep froggies clean, as it doesn't stick to their skin. Best of luck!