Not much beyond be really clear how much time you are giving them to 'discuss' among themselves. If it's ten minutes, remind them again at five minutes and make sure you have a good clear, LOUD microphone. Many of them probably won't listen but some of them should. Also when you are going to call them to order you could flip to a new slide. Nothing calls a lecture hall to order more than a new slide which they will all start transcribing furiously, in my experience!