"quiet signal" for those times (and they happen often...) when their students are too loud. Elementary students of all ages can get carried away by an activity, excited for any reason, and sometimes just want to visit with their friends. For these times, a quiet signal is necessary.
3, ...2, ...1
The one that I use the most is counting down from 3. I remind the students, "When I get to one, you're done" (talking, that is!) For situations when they are too loud and would not hear my counting, then I use the "turn off the lights" approach.
Another good one, especially in music, is having the students echo-clap a rhythm pattern that you begin. Also, I really like the chimes idea, especially in the music setting, my only problem with this idea in action is that I never have the chimes just where I want them when I need them!
Every teacher needs to experiment with and develop the quiet signals that work best for them. For more ideas, check out proteacher, teachertips/training, and teachercreated (good visual aids are also included on this site).