I was at the local park recently with my wife and 8-year-old son. My wife met an acquaintance who has a 6-year-old boy. The mothers got to talking about the frustrations and difficulties of keeping young, vibrant boys in line. The friend said, "I bring my child to church every Sunday. The only reason I do this is so I can tell him (when he's being bad), 'God is watching you right now, and if you don't knock it off, you're in big trouble." She said it works every time.
I was floored. I don't like to discuss religion, but I felt I couldn't just let it go, so I basically rephrased what she said in a question in a way that made it sound like I was flabbergasted.
From my perspective, I prefer to say, "I'm watching you right now, and if you don't knock if off, you're going to your room." Works every time.