When I am cleaning the house I tell the children that everything left on the floor will be thrown in the bin. They look at me in disbelief until I take action, pick up one of their favorite toys or garments and walk towards the bin. As I am about to throw away their precious belongings they scream:
"No, no, don't throw that away!"
"Well, pick up your stuff then, and put things where they belong."
It works wonders. I speak from experience.