This one works GREAT!!
If you keep asking your kids to pick up their toys and they just won't listen, tell them they have 5 minutes (a warning is only fair) to do so, or you will pull out "the bag."
Get a clean garbage bag and tell them that whatever they don't pick up, YOU pick up and therefore goes in the bag and will be stored away for as long as you see fit.
I have done this many times, and you should see how fast those little feet run around to get all the toys picked up. ha ha! I think it helps to use a garbage bag because they KNOW what usually happens to things that get put in garbage bags. (out they go!!!)