I have 2 tricks I do when I need the class to be instantly quiet or for when another teacher walks in the room and they need to be quiet.
The first week of school we practice this little routine. I tell them I have "Magic Fairy Pixie Dust" that will make them all go to sleep when I sprinkle it around the room. So I tell them "1, 2, 3, Sleep" and I act like I am throwing dust across the room. The students are taught to act like they fall asleep. When I say "Wake Up", they all raise up, stretch out their arms, and make yawning sounds. It is actually pretty cool as they all do it in unison. The other teachers think it is awesome. I simply say "1, 2, 3, Sleep" and the kids just love dropping their heads and closing their eyes.
The other is very similar, except I say "1, 2, 3, Freeze" and I act like I am shooting them with ice from my wrists (Sub Zero Style) and my students all freeze in place. Many will strike a pose like they are doing the robot or something. When I say "Un-Freeze!" they all start moving and make cracking sounds like they are "thawing out."
I know they are very cheesy, but the other teachers and visitors in my school think it is awesome and the students love "acting" in front of an audience, even if it is another teacher just walking into the room.
We spend a few minutes the first week of school practicing this, and I always have it in my toolbox when I need it. I will even have students tell me "You didn't ask us to Sleep or Freeze when Mrs. Smith came in!"