I love my little bell. It's the kind you press down on to ring, the type you find on store counters everywhere.
When I started working with small groups, volume control for the students working on their own was a problem. I let students know that if I rang the bell, that was their signal to quiet down. If I rang it twice in a day, the class would lose recess.
The lovely thing about this is that I don't need to stop working with my small group to quiet down the entire classroom. I ring the bell and a hush descends. I ring it a second time and there is a collective groan. I will occasionally issue a sharp reminder after ringing the bell, but I've realized it's not necessary. The bell is effective on its own.
One mystery of classroom management solved, 10,000 more to go ...