"Essential for a freed breathing is an upright posture, giving room for relaxed movement of all parts of the respiratory system. At the same time breathing also influences the posture. Most of the muscles part of the respiratory system are connected with the cervical spine and the lumbar spine. Thus breathing also has an influence on stability and position of the spine. At the same time the position of the spine affects speed and quality of breathing. Thus efficient breathing means correct posture and the other way round."
1. Use a Mirror
The mirror is a great teacher! It seems that I might be stating the obvious here, but you'd be amazed at the amount of useful information you receive from with a mirror.
The mirror will give you visual feedback on your progress, in real time. Using a mirror, you will become more aware of your habits. The mirror is especially helpful in monitoring the evenness in technique, shape of embouchure, and integrity of posture.
Here's a video for guitar students...still contains some great ideas!
The mirror will provide you with a visual perspective in your practice. Increased attention around your habits will make you a more self-aware artist.
2. Stand Up
It's much easier to take deep breaths and stay fully aware of your body in space when you're standing. Stand with a neutral pelvis, with the feet in a comfortable position, so the feet and legs can support your torso.
The lung itself is a passive organ, activated by movement in the surrounding structures (thorax and abdominals). The air stream entering your lungs is triggered by a pressure difference due to increased volume in the lungs. Naturally it makes sense to maximize your lung volume with an upright, open, and standing posture.
When you're free to use the full range of your lungs and abdominals, you'll take deeper breaths, resulting in longer and more supported phrases. You'll also have much clearer and more powerful articulation.