When speaking to a group, keep directions simple
Use the "When Before What" strategy: tell them when then tell them what.
Offer positive reinforcement to those participating and following directions ie: "I really like how so-and-so is doing such-and-such" or "Thank you so-and-so and so-and-so for doing such-and-such".
Stickers and other small prizes or gifts are a great way to show your appreciation and remember that a heartfelt "Thank you" goes a long way.
If it becomes necessary to offer criticism, do so using a compliment sandwich: one compliment--your criticism or problem--and a second compliment. Ilustrate for him/her/them a generally positive picture of the situation and allow the unwanted behavior to stand out.