Pee-Wee used to have a word of the day and whenever someone said that word, everyone would scream.
I've used this with great success. I let one kid pick "the word" for the day and whenever the leaders say the word, the kids scream real loud. When I need their attention I just make up a sentence with the word in it.
As an alternate, I use one word to signal quiet and one word where they can scream.
Switch words as often as necessary to keep it interesting for the kids!