Zach Whitman gave you some great advice, here's some things that I've tried with success for my reptiles that need more humidity (it's 30% here half the time normally!).
1) different substrate. If the snakes can take it, use a compressed (and then rehydrated) coconut bedding, but I've found this tends to be much too wet for most snakes (great for toads and/or more arboreal species). A good mix is usually 50/50 orchid bark and the coconut bedding.
2) Cover the top! Use plexiglass, books, whatever, but what I've done and had work exceptionally well is a damp towel (especially if the towel is dampened with HOT water) over most or all of the top. If you use a UTH, a damp towel over the entire top of the cage works wonders.
3) Fill a cup with HOT water out of the sink. Make sure it's steaming. Place the cup in the tank, cover the top, leave it sit. Obviously, if you're concerned about the temperature of the water, keep an eye on the animals. Faucet water is not usually hot enough to burn immediately, but microwaved water can be. Take it out after a little while, preferably with opening the top of the cage as little as possible. I used to do this when I had waterdragons, and it worked exceptionally well.
4) Of course, regular misting. But, fill up the water bottle with HOT water. If you are concerned about the hotness of the water, fill up the bottle, then spray it on your hand from about a foot away. At most, the mist is warm, not nearly as hot as it was coming out of the faucet. The warm mist raises the humidity much better than cool mist does.
5) Moss. Stick some kind of moss in strategic places in the cage, and when you mist, focus on the moss in particular. The moss will continue to evaporate and dry out and raise the humidity...and it makes the cage look pretty cool, too. Keep in mind the moss doesn't have to be sopping wet, just damp to the touch. I use this with my baboon spider, and decorate my cal king's cage with it. Looks nice dry or wet. 
6) Running water and live plants. These may not be the best things for a kingsnake cage, though. My calking has a live plant in her cage, but only as long as she is not big enough to destroy it. You may want to try a spiderplant or something low and bushy that would be harder for a snake to crush, but still be small enough to fit in a cage.
Good luck!
~jenny
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"Polysyllabism in no way insures that what you're saying is actually worth being heard." - Blake (an e-friend of mine)
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