My seminar instructor said something that always works is "Ladies and Gentlemen, thank you for your attention." She says it works amazingly. I've tried and have had okay results.
I found myself saying "Hey! Hey!" when my students got too loud. There used to be a TV show on in the late 70s called "What's Happening!!" When one of the characters, Dwayne, would enter they would say, "Hey! Hey! Hey!" with a certain intonation. (See the :32 mark here http://www.youtube.com/watch?v=OmxSOc1YH6g
). So I recently decided to use that as my attention grabber. I say Hey! Hey! Hey! and they all repeat it after me. They're 8th graders and you'd think I was giving them candy! They ask to say it over and over again.
It works for me because it fits my personality. I think the key is to find a catch phrase that they can repeat or respond to. It could even be a refrain from a popular song. Works for me.