In a tank? Cover the screen top with foil/plexiglass (there's a tutorial on here somewhere) or a damp towel, use a coconut-husk-based substrate, bigger water bowl, moving the water bowl to the hot side/ under the heat source, using a humidifier in the room where your snake is kept. That's all I can think of off the top of my head.

Some people get reptile misters/foggers. They're not particularly recommended for BPs, more for high-humidity amphibians and lizards such as dart frogs, but I've heard they can work as long as you put them on a timer to regulate how much they put out? Thing is you don't want the enclosure to be wet, just humid.