What are you using for a substrate? I use cypress mulch which holds moisture really well. Heat lamps kill your humidity. I have 2 younger snakes housed in glass tanks with screen tops with the entire top covered in foil with cut outs for the light (compact fluorescent) and a ceramic heat emitter. I mist 2 or 3 times a week. I've found it also helps to use terrarium moss in one corner of the enclosure. In the winter months when it's really dry I hang a bath sponge in the tank and wet it daily. They are super cheap and can be replaced or boiled if you're worried about bacteria.
I would also recommend a digital hygrometer. If you're using an analog (dial) hygrometer it is probably inaccurate.