5. How do I know that I am improving my flexibility properly?

... A good martial arts teacher will insure that his students seek improvement in their flexibility as a result of consistent regular practice, good warm-ups and cool-downs, and stretching that is longer and deeper by gradual degrees. It is more important to maintain flexibility on a daily basis and stretch intensively only occasionally. One should stretch only when the muscles are fully warm and to the extent appropriate for the level of activity performed that day. It is prudent to know your limits and wisely improve upon them.
... A good stretching exercise will create a sensation of sweet discomfort, not pain, and there should be a feeling of release in tension from the other areas of the body not directly involved in the stretch. It helps to smile, relax the face, jaw, and tongue, and wriggle the toes to ensure that you are not overly tense or causing damage. Exhalation while stretching is vital. Good breathing practice before and after a session of stretching will help the circulation of oxygen and removal of waste by-products from the tissues by the bloodstream.
... True flexibility is not measured by the degree of final stretch in an exercise, but is more effectively measured by where you start the next time. Even so, abilities from day to day will be affected by how intense the last class was (was there some new exercise?), outside activities (the Saturday softball game or the office picnic), your workday footwear (high heels or construction boots), sleeping position, etc. Factors such as the weather, humidity, room temperature, and even what you ate for breakfast can also enter into the picture.