For my DD I started some years ago telling her that I was going to clean her room, and that she could watch, or she could let me do it on my own. I promised not to take out stuff I knew was important to her and to put everything I removed in the shed for a month. If there was anything she noticed was gone and really missed then I would fetch it back for her. This morphed over time to "If you miss something we can talk about it."
I think the first time she insisted on helping and it was awful for everyone. The next time she stayed away and she's learned that I won't take things she will notice are gone and miss. She's never once gotten anything back. And she's much more able to keep her own room tidy after I have culled and done a bit tidy for her.
I have often done this late in the year, particularly if I am wanting to cull toys too, and told her that after the month in the shed the toys that were removed will be going to charity for the children who don't have as much as she does. She seems to like that idea.
