Unintentionally, I stumbled on a phrase this year that worked very well with my high school students (I also had large classes of Freshman). I would simply say, "Focus Forward" and then put my eyes on the clock. If students were not all looking forward and quiet within 30 seconds, I would begin to write the time on the board in 10 second increments. Just the idea that I was writing time on the board would grab everyone's attention! Only one time did I get past 60 seconds. Students would ask me what the time meant but I never told….just would say, "You really, really, REALLY don't want to know!"
Some teachers do use the 'watching the clock' method and hold the students extra time (this depends on your school's policies), or give more homework (doesn't work for students who will not complete HW anyway), or give a quiz if they reach a certain amount of seconds.
For me, I don't want schoolwork or homework associated with punishment. I did have a contest during a very challenging week (right before Spring Break) in that I kept running totals by class periods on my whiteboard. The class with the least amount of 'seconds' earned a reward day. That worked very well - so well, that I will use it again this year!