Stability & concentration
Regardless of meditation technique, it is important to start with a balanced posture. One of the easiest ways to work on a stable posture is to bring three points/regions of the body in contact with the mat/cushion.
— If one can sit in a cross legged position, the three regions are our two knees (or Vastus Lateralis to be more precise) and, depending on how we sit on a cushion, our coccyx (tailbone) or the sit-bones (two bony areas protruding at the bottom of our hipbone, Ischium). If one can not get the two knees to touch the mat, one could use small supporting cushions to prop up the knee.
— If one can sit on a mat but not cross-legged, one can sit in seiza posture while using a seiza bench or two stacked cushions to elevate the body. This posture can bring a lot of pressure on the knees.
— If one is sitting on a chair, it might be very helpful to keep a cushion under our feet such that knees are at a height above the hip region and one doesn’t slide downward during sitting.
In addition to stability of the body, some meditation traditions give a lot of emphasis on maintaining a straight up and elongated spine such that the healthy curvature of spine is supported. Zen tradition asks us to imagine sitting as if there was a chord going between one’s crown and the ceiling, and another chord going up from one’s chest to the ceiling. One also tries to maintain the curvature of the spine such that lumbar area is curved inward and hips are protruding outwards (and chin is tucked in lightly to elongate the cervical area).
Overall, our posture should support deep breathing from abdominal region (called Hara/Dantian in Japan/China). Breathing from hara brings in more oxygen to the body and innervates vagus nerve.