The holes you have for venting are perfect, in my experience humidity problems just come hand in hand with an aquarium. However there are ways you can try to keep the humidity up. You could try a bigger water dish. Have you thought about the DIY humidifier we have in the DIY section? It works pretty good, after you tweak it a bit. By tweaking i mean getting another human heating pad and wrapping it around the bottle to keep the water warm. I've used it successfully. I've also make my own out of a regular humidifier with a pipe from it to the tank and a heating pad under it to keep the water warm. What are you using for substrate? You could switch to something that holds humidity better, like some cypress mulch.