Non-verbal cues are the best because it causes very little disruption to the learning environment and other students. Often times, off task students are constantly watching to see if the teacher sees them so I use this opportunity to keep teaching, but wait for eye contact and BAM, they're caught, and go right back to work. Other times, I use proximity to re-direct students. I rarely stand in front of the room teaching. I am usually walking between desks or standing near students who appear off task. I also point to the assignment at hand to redirect as well. I have found non-verbal cues to be more effective than verbal cues and helps to maintain a calm learning environment.