An old trick from a school assembly presenter (no microphone required) for grades K-6:

Put your own hand on the appropriate part of your body as you say the following:

"If you can hear me put your hand on your shoulder.
If you can hear me put your hand on your chin.
If you can hear me put your hand on your head.
If you can hear me put your hand on your nose.
If you can hear me put your hands in your lap."
Even if only the first row of children can hear you, the students behind them feel like they're missing something, so they quiet down and join in. The room is usually silent by the time you get to the hands in the lap, but you can continue adding motions until you have their attention.

Pulling out a large super soaker also gets their attention, but does nothing to make them less unruly. And teachers have told me that pointing a gun (even a bright green and orange plastic water gun) at their students was in very bad taste. Interesting observation: pulling out the water gun increases the ambient unruliness only until you use it, at which point it peaks. (Was it Ibsen who said that a gun onstage must go off in the second act?)
Chiisuta replies "I think it was Chekhov's gun." Hmmm. Maybe Chekhov said it, but the guns in question are definitely Norwegian.