"Stop for just a moment-look at your back foot to see if your toe is facing straight ahead. The toe must be straight ahead to stretch effectively." Or, "I see people having difficulty-let me demonstrate what I want you to do."
During a wall stretch for the calves, give the following instruction to people experiencing difficulty: "I would like to turn your foot so it is straight. Is that OK? Can you feel a difference in the stretch?" Always ask permission before touching a student.
Stand beside the person having trouble and demonstrate what you want him to do. Perhaps he cannot quite see or hear you well enough to comply. If the person you are correcting is down on the floor, get down next to him to demonstrate. A person on the floor is more vulnerable than the person standing, so you must get down on the same level to instruct in a nonthreatening way.
If you stay at the front of the class, only the people in the front row will be able to observe your technique. If you are teaching step, put several benches around the room so you can move around during the cardio segment. Try teaching in the middle of the class instead of the front or regularly move from the front to the back to the side of the room.
Most people respond much better to positive rather than negative reinforcement. If a participant is having difficulty with a movement or series of movements, point out someone performing well in class for her to watch or pair them together. Always demonstrate and instruct good alignment to keep a focus on correct technique.
Compare "You must have your foot in this position" with "Place your foot in this position because it will prevent you from falling forward and will make this exercise easier." Or, "Don’t bounce while stretching; that’s the wrong technique" versus "If you bounce while stretching you might pull or tear a muscle-I don’t want you to get hurt. Try holding the stretch instead." Which statement would you rather hear? Both statements tell the participant how to correct their actions; however, the second statements include a rationale and helpful alternatives.
Words such as good, bad, right, and wrong are emotionally loaded and judgmental. Instead of saying, "Joe, you are doing this movement wrong," try, "Joe, you seem to be having trouble with this movement. Let’s try this . . . I think it will help."
This is an excerpt from Methods of Group Exercise Instruction, Second Edition