I've kept mine in loose stacks, sometimes in hard plastic (200 count?) card cases and have never had any problems. Right now most of mine are in various unprotected stacks around my room (even Magical Chase). Still, I recommend SeymorOnion.com 's HuCard sleeves. They're better than the official sleeves, because they're new and clean and consistent and we know for sure that they don't have any kind of harmful acids in them.