You can always go with the "10...9...8...7...6...." routine. Usually works for me, but then you need to speak clearly and fluently to keep their attention. Speak (really) softly to allow them to open their ears and listen. I work in an inner city high school and this works all of the time.
I have rules set up (paraphrased: 1. Respect everyone. 2. Listen, not talk, while others are talking. 3. Discuss ideas in an orderly fashion.). So, I'll say, "I need you all up here" and use two of my fingers to point at my eyes. I wait a moment, and if anyone's still mumbling, I'll politely say, "<student's name>, which rule are you breaking?" Because I've established the rules so well (they've been tested on it; we've gone through them millions of times), they'll usually roll their eyes, say, "1 and 2," and then I'll respond, "Thank you." After that, the class is pretty quiet.
Some people raise their hand and have their students raise theirs in response until the whole class is quiet with their hands up. I don't think I could pull that off because I have a fear of holding my hand up, and no one caring
I don't do anything fancy--I just make a general statement about how I need the class's attention. I wait until ALL students are quiet before I start talking. I will never talk over students, and they realize that quickly.
Stand in front of the class with your hand raised and say something simple like "I'd like your attention please", and wait. I haven't tried this myself, a highly experienced teacher told me this and she said it really does work.
With 6th graders, they totally mastered my favorite technique, in which I say "I need all eyes on me" and wait to continue until I have made eye contact with every student and there is silence. Don't know how it would work with high schoolers, especially so many of them, but it's something to try.
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! .
I loved that show. I am going to have to try that one out!
Hey! Hey! Hey!
I just stand there and say, “That’s OK. I’ve got all day. You, on the other hand, have the same amount of work to do whether we start now or in 10 minutes.” Once that cat is out of the bag invariably there’s shushing and eventually a “Hey, shut up!” A few acrimonious looks are exchanged and I offer to wait some a couple more minutes if they need me to.
My students police themselves pretty good. By the time we’re 4 to 5 weeks in all I have to do is stand there and look like I want to say something and it’s like someone is knifing tires in the faculty lot. “Shhhh!” “Shush!” “Jeeez!” “SHUT UP!”
This technique also works during a note taking session. I don’t do lectures. I give you info that you really should have on your notes because I like open note quizzes and since I’m a Professional Technical Educator I like to let you take your notes to your machine when I give a low stakes practical examination. Sometimes, during these sessions I’ll find a couple of young people with their heads together talking or simply engaging in a little game of what I call “Slap & Tickle”. Basic goofing around stuff, they get all middle school and giggly. That’s when I engage a “Proximity Teaching” strategy.
Proximity Teaching means that if a couple of students are off task rather than hollering at them I simply move myself toward them. When, in proximity, I get close enough they will simply be quiet. Yeah, the teacher is standing right there teaching—whatcha gonna do?
Stand in front of the room, look impatient and unimpressed, and they will shut up. All it takes is one student to see you and they will signal to the rest of the class.
I count in my head when I do this and I'll let them know how long it took for me to have their complete attention. This works for their competitive side - compare them to other classes or to their previous bests.