I've found several keys to maintaining higher humidity: First, having more surfaces that collect and hold spray so it evaporates more slowly. In other words, the more surfaces (leaves, twigs, plastic sheeting on the cage sides) that get wet, the longer it will take the cage to dry out. My cages are really crammed with live plants so I can hardly find my chams. The only area that is open is right at the top at the basking perch. Second, a finer, lower intensity misting for a longer period works better than a heavier blast for a short time because it gives all those surfaces a bit more time to get thoroughly wet before excess water runs off.
The humidity level in the room your cage is in plays a big part too. If your heating or AC keeps the room dry (and that is usually more comfortable for the humans living in it unfortunately), your cage is just going to dry out faster no matter how much you mist. Adding a room humidifier that either runs near the cage or is piped into it can really help a lot in between misting cycles.
Oh, and one more thing...actually the bigger your cage is the more you can stuff it with LARGE leafy live plants! Larger cages just hold more moisture. My best cage for holding humidity was 4'x5'x5'.