Some people will use a mixture of spahgnum peat moss and dirt or a soil and sand mix to keep the humdity up. Normally I see Savs kept on cypress mulch to keep the humidity up, but most prefer the dirt mix.
How high should the humidity be? I keep finding mixed feeling on this topic. Some books/site say they need about 50-75% others say as long as they have a tub of water large enough to soak in, don't worry about it.