We say it all the time to the kids.... "Quiet please" - why not do it as well? This is a simple classroom management technique that I like. Basically instead of the typical 'compete with the kids for silence'; the conversation that usually goes like this...
"Ok.. Johny.. sit down... Stephen... stop talking... Mike... put away the phone... Clarise.... stop talking..."
Furthermore it usually takes place while the teacher is at the front of the room. The result is for the kids in the back row, the teacher is 'all the way up there.'
Try walking around the room and non verbally tap students on the shoulder... remove distractions... give non verbal gestures for silence. A brisk walk around snap fingers for attention, make non verbal cues, and so forth.
It saves your voice, generally gets the same results and I believe walking around the room gives you a bigger presence in the class. I also believe in the old mantra... noisy teachers have noisy classrooms.