Here are some photos of various people demonstrating good (and not so good) use of themselves.

 
Sitting in an over-straightened manner Sitting in good balance Sitting all the way back in the chair (slump).
The head is going backwards and downwards on the spine, and the lower back (lumbar) is curving forward in an over-contracted and exaggerated way. The chest is slightly held up. The weight of the body is somewhat in front of the sitting bones into the legs. Besides being very tiring and full of tension, sitting in this way greatly compromises easy and natural breathing. The head is poised over the pelvis, releasing forward and up off of the top of the spine. The lower back (lumbar) is not over-curved, but is releasing back and up into its natural curve. The weight of the torso is taken through the sitting bones. Sitting in this manner allows for comfort, healthy breathing and an easy sense of balance. Though the torso is ostensibly “supported” by the back of the chair, it is actually pitched backwards, which puts the head too far forward. Because of this the head is no longer balanced and supported by the lengthening spine. Instead, it is being held up by too much work from the muscles of the shoulders (and the neck itself), creating undue strain. Also, the shoulders and arms are pulled forward, narrowing the torso. None of this is good for free and natural breathing.
Standing in an over-straightened manner Standing in good balance Standing in a slump
The head is going backwards and downwards on the spine, and the face is pointing upwards. The curves in the neck and lower back are over-exaggerated and tense. The knees are thrown back and locked, making the legs, hips and lower back unduly stiff. The chest is slightly raised, making the upper torso rigid, compromising breathing. The head is releasing in an upward and forward direction on the top of the spine. The legs are releasing the heels downward toward the ground, creating an oppositional balance with the upward lengthening spine. The curves in the neck and back are natural and easy, the back staying back and releasing upwards. There is freedom and balance in the torso, allowing for optimal breathing. Most of the muscular direction is downward. The head is being dropped from the base of the neck, thus losing the support of the spine. This puts undue strain on the shoulder area and the neck itself. The chest is collapsed, compromising breathing. This downward direction also puts undue strain on the lower back and pelvis are, which compensate by tightening and narrowing.
Sitting in a chair that is too low Sitting in a chair with something firm (zafu meditation cushion) to make a better-fitting chair  
 
The knees are too high in relation to the pelvis. (The angle between the torso and the legs is a little less than 90 degrees.) This causes over-straightening of the lower back (over-exaggeration of the lumbar curve) and too much effort in the hip flexor area (where the upper, front part of the legs joins the pelvis). The sitting bones are too far back, placing much of the weight of the head and torso on the legs. The knees are lower than the pelvis. (The angle between the torso and the legs is about 120 or 130 degrees.) The weight of the torso and head is balanced over the sitting bones. This allows for the back to release back and up into its natural curves, helping to support the head on the top of the spine.