I can always tell when the girls have outgrown their toys. They start fighting more and whining more. Last week I heard "there's nothing to play with." I thought, "really, I can make that happen." And then I decided it was time to clean out some toys and introduce some new ones.
Each holiday I put away a few toys. When they grow bored with one, I'll switch it out for another one. Since Christmas is coming, I purged all the toys they've outgrown, broken, etc. Ten brown, paper grocery bags later, the room was much cleaner. The grocery bags went to a local resale shop. The rest went in the garbage.
The girls seem much happier with less stuff. I'm happier that the room seems a bit neater. And, neither girl seems to notice that Mom purged their stuff.