Here are some of the tools and techniques I used in our recent household re-org.
I got these shelf dividers at Walmart. We use them to help with our kitchen storage. My son had these "display" type items taking up space in toy boxes and they didn't make good use of this tall book shelf. With the dividers, everything fits and he didn't have to give away any of these "treasures."
I am big on boxes or "homes." Everything needs a home. If things don't fit in their home, then maybe we have too many. We use smaller boxes to hold smaller treasures, but the rule is that the boxes need to open and close easily. If not, they are too full. I also have a paper mache treasure box that we use for what I call "junk" - the things that come in treat bags, meal toys, etc. Once the box is full, things have to be thrown out.
The final thing that worked well this time around was a physical boundary. I had decided that all of my son's craft supplies needed to be in one spot, preferably in the kitchen where he can be supervised and where we do most of his crafts anyway. These boxes used to be filled with puzzles and some craft things. I said everything had to fit in the three boxes designated for crafts. That forced him to make choices about what he truly wanted to keep. We did the same thing for toys in his room. They had certain homes and everything he wanted to keep had to fit in those spaces. It worked beautifully!
So pleased with how the project turned out, even if it did consume my entire vacation from work and several other days as well.