It's a nasty fact of ballet...your feet are going to sweat. To keep your pointe shoes in good shape, it is necessary to allow them to dry/air out between practice sessions. After a tough class, don't stuff your damp shoes into your dance bag. Instead, place the shoes into a mesh bag (available at most dance shops) and allow them to breathe on their own. The air will dry the moisture and help them to stiffen again. To keep them smelling fresh and clean, stick a deodorized dryer sheet into the bag with your shoes.