I have used "buzz" rings very effectively. When the group here's the rings going and sees others begin to gather around, they all start to join in.

Two effective verbal methods are: 1. "Clap once if you can hear the sound of my voice.", then clap. Say this clearly, but you do not need to shout. People nearby will hear you and join in. "Clap two times of you can hear my voice." a bit softer. Continue softly as more people begin to respond. it is sometimes fun to see how many times the group needs to clap before you have everyone's attention. 2. Shout "Marco" and wait for the response (which is "Polo" in case you did not know that. Continue more softly until you have everyone's attention.