I have a similar problem in winter when our heating system destroys the ambient humidity in the house. I recently came up with the ingeniously obvious solution: Get a humidifier for the room that your snakes are in. It has done wonders for me, and now my little Caspie actually comes out of her hide at night, looking much more content! And even when the humidifier is up pretty high, the room is still comfortable for me to be in. Even if the humidity doesn't go up to where you want it to, you can "supplement" with spraying the tank down a few times a day.

(By the way, if you see "dents" in your snake's eyes from low humidity, don't worry, just give her a nice long soak and she'll be good as new.)