These are some important internal martial art's principles taught to me by Sifu Mathews. I have found them to be the most valuable concepts to observe if you wish to develop a strong and graceful form.
Three Internal Unities: Xing/Yi --> Chi --> Li
Mind/Intention (Xing/Yi) moves Energy (Chi).... Chi moves the Body (Li). Each movement should start with an intention, then allow the mind to distribute the energy with maximum efficiency to accomplish the intention... then allow the body to move according to your trained skills by means of maximum energy transfer.
Three External Unities: Shoulders/Hips - Elbows/Knees - Wrists/Ankles
The shoulders and hips should traverse in the same general directions and should not oppose each other's movement. The Elbows and knees should also move in the same general direction with out opposition. And finally the wrists and ankles should be considered as to their general direction of travel. All of these principles tie together and are really the same concept of maintaining proper alignment. The first two external unities are the ones to pay most attention to.
Three Bows: Torso with Legs - Arms with Torso - Legs with Kua (Groin Area) Open
All of the postures and transitions should maintain a bowing of the torso arms and legs. A proper examination of the still Yang Cheng Fu postures would reveal this bowing. Another consideration is to make sure as you transition from one posture to another, that these bows are maintained to the best of your ability. The torso should always remain rounded with the upper/lower back straight and the pelvis tilted upward. Elbows should always stay low, but not collapsed to the body... arms are rounded with the relaxed shoulders and sunken chest. The knees should always strive to bow outwards, but do not over bow and put too much weight on the outer edge of the foot. Really watch the leg bows during transitions. Again, these principles all rely on each other for continuity. If you loose one bow, you will probably compromise another.
I welcome your responses on these important concepts.