The best technique I've ever seen used for getting the attention of elementary school aged kids is one I see the vice-principal use at school assemblies. When the children are becoming unruly he stops talking and begins to clap in rhythm. The children copy what he does. For example he may slap his thighs twice in quick succession then clap his hands together three times slowly. After about two rounds of this, every child in the auditorium has quit talking and is paying attention to him. Then he can continue his presentation. This is the best use of child psychology that I've seen yet. He speaks not a word, yet captures the attention of every child in that room.