I love everybody's descriptions of cleanup madness!
I do the 30 second countdown..sometimes the numbers go slow, sometimes I speed it up when they aren't exactly on the job...and if there are a few kids at the bitter end of the count really getting stuff in order I will go to 1...1/2...1/3...1/4...to stretch the time for them!
Works well. I am consistently 'a day late and a dollar short' with my own time management, so I know how it is not wanting to stop everything. Kids respond well.
I also have the 'pick up 10 scraps' when it is a cut paper project, that really is magic, as someone said.
I taught my first graders for the first time yesterday, new crop of kids, and OMG...well, let's just say it's going to be a lonnnnngggg year!