How do you keep humidity up for a ball / royal python?

Apart from spraying the ReptiBark substrate regularly and providing a large water bowl for your python to bathe in, you can put a large clump of Sphagnum Moss in the cool end of the vivarium and keep it quite damp. See :

http://zoomed.com/db/products/EntryDetail.php?Entr...

This should maintain sufficient humidity and allow her to shed properly.

Get substrate that will hold in humidity well such as eco earth or even paper towels hold in humidity really well. Because that cage is big, you are going to want to also spray the cage with water just to keep in the humidity better.