I had that problem. I got tired of it and told my kids that they have to put things away after they are done playing with it. At the end of the day, if there is anything left out...I get rid of it. They know this rule and it works. There are times when I end up tossing out a bunch of stuff. My thought is if you can't take care of it then obviously you don't need it.
It is on the extreme end of things, but it works for me.