This Friday the kids are out of school. They don't know it yet, but I plan to have them pull all the furniture out of their rooms so I can shampoo their carpets. Due to the size of my house this will have to be only one room at a time, so it may take longer than just Friday. But, every scrap of furniture and all other "stuff" will be coming out of their rooms. I'm then going to have them go through every single one of their belongings as it goes back into their rooms to weed out what they can. Outsized clothing that was hiding in the bottom drawer, scraps of paper that lay forgotten behind the bookshelf, bits and pieces of broken toys hidden in various nooks and crannies. The whole thing. All of it. Every last piece.
With the amount of bitching and moaning that they do for just straightening up their rooms, this activity should scar them for life.
Excellent!