First, a few words of advise. Don't say it if you don't mean it. Kids know when they are being played. It is the sincerity of the Magic Words that gives them their power. Also, say it like you mean it.
Intention is everything.
Then, and this is most important, listen to what they have to say. Encourage suggestions from them. Listen some more. Let me know your experience with the Magic Words. My husband was complaining about some of the students in his physical education class not participating, as he would like.