My DD is 3 1/2 and one thing I've found really useful is periodically sitting down with her and going through all the toys and stuff in an area. I let her decide what should be packed away, what should be donated for another kid to enjoy, what should be thrown away and what she wants to keep out. We put the things in four piles and she is GREAT at both doing this sorting and deciding to only keep what she wants. With stuff she's on the fence about but I think she doesn't really play with, I convince her to pack it away and promise to get it back out if she decides she wants it. Now, the only toys in our living room are some dress up stuff, "dolly", musical instruments, and books. In the playroom we have a doll house, blocks and art supplies. In her room, stuffed animals and books.  I've been actively doing this with her since she was 2.Â
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DD is also the ONLY grandchild on either side of the family and she gets inundated with gifts of all sorts. A few times I got angry with my parents about this and they stopped giving her huge piles of stuff. Now they give her little toys when they see her (that often get put in the trash pile a few weeks later) and they ask my preference for big presents for Chanukah and birthday (Train set, membership to museums, etc...). My in-laws consistently bring 12-15 items for DD. DH told them not to bring plastic garbage because they were ruining the planet one time in a fit of anger so usually it is books, toys of some quality, or art supplies.Â
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Despite these requests, she still gets WAY TOO MUCH. So, in addition to the sorting we do together, I have embraced the idea that I can't control what people give her but I can control what stays in my house. I routinely donate items that DD has never opened or played with. Sometimes she gives her ok, sometimes she never even noticed the item because it's in the midst of so much other stuff.Â