Read my colleague Stan's blog Chi For Living - Searching for the Perfect Position for a very good recommendation for those beginning Tai Chi or other mind/body workouts.
I reinforce this idea with my students, & ask them not to copy me, but rather guide them with how it should feel to them.
Everyone has a different body structure, way of moving, mindset, personality. So why should we all look the same, especially when doing Tai Chi or Yoga?
Of course there are guidelines, but perhaps this style of movement is best learned from the inside out. Feeling the move rather than just imitating the teacher.
It reminds me of one time when I was teaching an aquafitness class and needed to scratch my ear.
Almost everyone in the class copied my move, since they were so tuned in to copying the exercises.
In this respect, when teaching, I try not to focus too much on the technical aspects of a move at first, but rather I start with verbal cues such as:
"move your hands as if they were resting on a balloon floating up" or "bounce a basketball in front of you in slow motion" etc. Imagery is very powerful and helps a participant to use their own experiences to move properly.
Another helpful tip is if someone has difficulty in coordinating the arms and feet moves at the same time, they can be learned separately with a positive effect. In fact they may get 'soong'(Chinese for relaxed with soft structure) much faster and get the benefits from the class that much faster. Too much input creates tension both physical, and mental, and thus decreases the energetic flow as well.