Try covering a good deal of the top of the tank. You can just use a towel until you figure out how much you have to cover to get the humidity right; but if you are using a glass tank it will probably take a lot of covering to do the trick.

Check out this thread - On page 1, Argentra and Anatess have posted pics of their glass tank setups where they have done just that (there might be others later in the thread as well.)
http://www.ball-pythons.net/forums/s...ad.php?t=83055