How to get the attention of unruly school children

Every time I open my mouth an idiot speaks.

This wasn't an isolated incident either. Another time at assembly he said:

Boys will NOT play with their balls in the covered area. Otherwise a member of staff will approach them.

The best technique I've ever seen used for getting the attention of elementary school aged kids is one I see the vice-principal use at school assemblies. When the children are becoming unruly he stops talking and begins to clap in rhythm. The children copy what he does. For example he may slap his thighs twice in quick succession then clap his hands together three times slowly. After about two rounds of this, every child in the auditorium has quit talking and is paying attention to him. Then he can continue his presentation. This is the best use of child psychology that I've seen yet. He speaks not a word, yet captures the attention of every child in that room.

She said that those who could hear her had to stick out their tongue. Some kids were listening, so duly complied - and those who hadn't been paying attention asked what was going on, and did so too.

And it worked. The entire class shut up, and as a bonus, they were all looking pretty stupid.

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."

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.