Humor helps as the "Teacher Mistakes" points prove. Don't be afraid to laugh at yourself. And maybe, in the middle of directions, add "and the first student to stand and jump on one foot gets a (candy bar, pizza, gold star, whatever)". One or two will hear it. The rest will start to listen.

Never talk down to anyone! Animals, children, elderly people, people with English as a second language, learning disabled, etc. respond best when you speak to them as adults.

Keep directions simple and direct. You probably have their attention initially but minds wander.

Rules? What rules?