As far as the hoarding thing, DD got extremely mad at me about smuggling things to the trash, and I get it. She now has a plastic box under her bed for papers/keepsakes. If she wants to keep it, it goes in the box. If the box gets too full, she must sort the box and discard some items. (I have told her that as she gets older, we MAY eventually get another box.) It is NOT a big box! This has helped a lot, as she has to make the call on what is worth the space, but she no longer worries that I will throw things out on the sly.
