I try to watch and see who has taken care of each item. Once they are all done (or we are out of time), I tell the students to return to their seats. Then I announce the winners who each get a piece of gum or candy.
I especially love the first time I do Mystery Trash with my students! I pick a couple things like a small piece of paper on the floor or a pencil that was dropped, but then I always pick something like a book that was crooked on the book rack or a tissue box that wasn't perfectly in the corner. They never get that last item on the first day, but after I announce the winners I say, "And the last Mystery Trash item that no one got was...this book! Look at how it's crooked on the book rack! We can't have that!"
The kids' eyes get wide and they say, "What?!" They can't believe I'm that picky! But you should see them clean and straighten the next day! It's a beautiful sight!