Common sense and awareness are the key ideals in these guidelines and cautions.
You must be able to move freely so comfortable clothes, bare feet and empty stomach are in order.
Even though you should do some warm ups and deep breathing before starting your practice the room should be well ventilated but not hot or cold.
It is more important to do fewer poses slowly, gently, smoothly, with awareness and carefully than to do more done hastily, straining, pushing. It is safer and more effective to ease into a pose with awareness and breathing than forcing. Honoring contraindications regarding the poses and breathing exercises is essential common sense.
“Never compete: Don't expose yourself to injury by competing with others--or with yourself. Pride of body or of superior flexibility have no place in yoga. Progress in yoga is not absolute, but directional--it's a matter of taking your own next steps.”
The usual way of breathing during an asana is diaphragmatically.
“In all poses, avoid swayback (over-arching the lower back) by tucking the tailbone when necessary to lengthen the lower spine. In some cases, contracting the lower abdomen seems a more natural movement and will accomplish much the same thing.”
Keep the neck in line with the rest of the spine so compressing the cervical vertebrae or discs is avoided. “Avoid tensing the shoulders; keep the back of the neck extended and the shoulder blades released down the back.”
“Avoid twisting your knees; they're hinge joints, made to fold with little or no rotation.”
In standing poses don’t lock the knees (don't push back). Instead pull up the kneecaps by moving the quadriceps or slightly bend knees.
For bent knee standing poses, when standing on one leg “never allow the knee to go beyond the ankle. Keep it over the ankle (lower leg vertical), or for extra protection, slightly behind the ankle (i.e., don't come down so far into the pose).”
In forward bends keep the spine lengthened and bend at hip joints. “It's okay to let a healthy spine round slightly, but only through relaxation and with complete awareness, a long spine, and no discomfort. Avoid entering or exiting the pose with a rounded spine and straight knees, as this can compress the intervertebral discs and pinch the spinal nerves. For the greatest degree of safety, keep the knees slightly bent throughout the pose.”
To protect the lower back (lumbar) in back bends tuck the pelvis (elongate the tailbone downward), release the shoulder blades away from the ears, lift through the sternum and allow the curve of the neck match or be less than (if your neck needs support) the curve in the rest of the spine.
To keep the breath open and not overly stretch the spinal lateral flexor muscles keep the underside of your rib cage open in side bends.
Remember to keep the spine lengthened as you exhale into a twist. If the spine rounds back off to prevent spinal compression of discs and nerves and keep the energy flowing freely.
For inverted poses never stress the neck and keep the natural curve of the cervical spine. If you are not strong enough then practice simpler inverted poses.
With sitting poses with crossed legs let all rotation come from the hip joint, not the knee. “The lotus pose, or even the half lotus pose, is not for beginners unless their hips are already very flexible.”