Since I've had this boa in my care, I have had to mist the cage, soak the substrate, and even had an aerator set up to bubble the water in an attempt to keep the humidity up. During the holidays I received an odd looking humidifier as a gift. It's basically an oddly shaped bowl with a tiny lid in the middle. The water is electrified near the "arc" and a rather eerie looking cloud of water vapor slowly flows over the sides of the bowl.
The top of the cage is wire mesh, complete with my very best friend--lid locks. We have a ceramic heat lamp that sits upon the mesh as well, but at a reasonable distance. There are some low branches for her to climb and I don't want her to get accidentally burned.
I have a towel that covers half of the cage, and on top of this towel is the humidifier. I've modified the bowl so that most of the water vapor trickles into the cage. For the past few days, I've seen her emerge at night to bask. Rather than face in the opposite direction, she faces the stream of water vapor. I'm happy that I can now turn off the aerator, as I'm sure that she didn't enjoy the vibrations one bit. I turn the humidifier towards or away from the top of the cage in an attempt to maintain a steady humidity level. For the past few days, the humidity as lingered around 70%.
Later on I will attempt to modify the humidifier (just the bowl--not the electrical components) so that it can reliably keep the tank at about 70% all of the time.
Keep a good sized water bowl in the tank near the heat source. Keep your substrate dry at all times. If you feel it necessary to have more humidity I would recommend creating a humidity box out of a tupperware. Cut a hole in it and place moist paper towel or moss. The snake will go into it when it needs more humidity.