It's called more water. Without knowing the details of your setup, try these ideas:
Add more live plants.
Mist more often by hand...slowly and for a longer time.
Add an automatic misting system controlled by a timer.
Offer lukewarm showers (put a plant in your shower, aim the sprayhead to bounce off the wall so the plant and cham get it secondhand, and give the cham a 20 minute rain).
Use a room humidifier aimed right at the cage.
Hang some plastic sheeting on the back or sides of the cage to help hold in moisture.