Willow may not be the best choice for a humid enclosure but if you like it, then it's up to you I guess. The best way to "treat" it would be to let it sit in your oven set to 250 deg (cooking time varies due to elevation) for about 2 hours. That is for hard woods, I would go about 200 for 2 hours with a soft wood like willow. For the leaves, it depends on what you are doing with them. Are they going to be a primary substrate or just the covering to the main substrate? Are they willow leaves or hard wood leaves? This is important because soft wood leaves tend to get real mushy (that is a scientific term)and rot quickly in a humid cage. If your useing hard wood type leaves, crumple a handfull up and lay it down first, then add a few larger ones on top of those - this will create micro-habbitats and help in feeding any plants you may have planted... Just remember to add more in the fall. There isn't too much you can do with leaves to clean them, just make sure they are collected from an area free of pesticides and check them over for any bugs before adding them into the cage.