If I were going to mention only one thing, or the most important thing, that I believe that anyone who practice Tai Chi Chuan should focus on more than anything else, and would ultimately lead to success in the art, well, that would be body awareness.
What we do in Tai Chi and why we focus on practicing slow, focused with internal awareness is for developing control, fine and refined motor skills. The kind of fine motor skills a Tai Chi artist wants to achieve is the kind of skills dealing with small details of painting, or like a surgeon operating with a laser. There’s an absolute precision that we are looking for. When we can control our movements in an absolute manner and with awareness, then the results will be a spontaneity and freedom of movement.
Therefore, when we practice form slowly, we should try to be aware of every slight tension that might occur in our body. Sometimes, or often, it’s good to move really slow so we have time to feel what is going on inside and throughout the body, from toe to fingertip, from the bottom of the feet and up to the crown of the head. Try to feel what ever slight change of movement does to your body. When does tension occur and where are there weaknesses in your movement? Move in an absolute even pace. Pay attention to the evenness of your movement at every inch of your movement. When the movement is straight, it’s perfectly straight. When it’s round, the shape is perfectly even. Perform your movements with the same perfection as you were a surgeon operating with a scalpel or a laser.
This is in my opinion “correct” form practice. Drills and push hands can be practiced with the same kind of focus on detail and internal movement. This kind of practice demands extreme focus and concentration. Yet demanding, it’s actually not very hard to achieve with practice. Just know how to practice and continue to practice. A certain kind of awareness should be developed throughout the whole body. Also, calmness of mind, and a certain natural beauty of body movement, could be the result. If you look at professional dancers, they often move with a certain authority or beauty that looks perfectly natural. If you look at “masters” of different martial arts there is often something similar. Sometimes when they enter a room, there might be the impression just like that their mere presence takes up the whole room. I believe that this is an expression of moving with awareness, a type of body awareness, that is achieved through many, many years of practicing with awareness.