What is doctrine on ordinary daily posture?

Funny you ask this, I was just at my orthopedic today for a right shoulder issue and was talking with him how my physical therapists guidance on controlling my posture has helped my minor SLAP tear and stuff improve. I still need to carefully workout to retrain the surrounding muscle and tissue but yeah now I am conscious of it and the cause... years of bad extended posture.

Buddhist/Eastern advice:

• Buddhism has the various sitting postures. They are all conducive to better lower body health and bioenergetics. The highest posture is considered the lotus posture which one should perfect. The Buddha talks about its benefits for chi transmutation extensively somewhere and there are entire Buddhist schools that center around this like the Lu Mountain Temple here in Rosemead, California where they bear the pain until they cannot any more because it is good for one's karma. Taken from Tao & Longevity by Master Huai-Chin Nan:
• Although some people may suggest sitting for hours at a time I advise against this. I suggest to intersperse just a few seconds of yoga if you will be sitting for long periods of time as modern science suggests that one's organs begin to shut off just from 20 minutes of sitting down. Buddhist walking meditation is also a good alternative, too. My sessions tend to be 20 minutes in lotus posture or standing meditation and then a few minutes break involving yoga poses and the five tibetan rites.
• Taoism suggests to sit maximum of 8 hours, lay 8 hours, and be on your feet 8 hours. This is recursive of course and a tally of one's daily total.

Worldly advice:

• When working on your computer be conscious of your elbows because they affect your entire ergonomics! High-held elbows will tighten your shoulders and put excess strain. Push your chest out and relax your shoulders. Try to sit in a relaxed erect position just like how when one is in lotus posture. There should be a J curve at the lower back.
• Try to use a sitting ball as a chair instead, I used to hate it and then got used to it over time and now I love it and it is great for your back muscles!
• Get a standing desk
• Alexander technique has some great advice that is visualization based and informs one's body posture

I have a very vague memory of a sutta in which Buddha mentions graceful posture, in passing. Somehow in my memory, he specifically commented on someone's manner of sitting down. He said, one should sit down slowly, not crashing down. This may be a fake memory though, could come from another source, not sutta.

Buddha also gave specific instructions for behavior during an alms round. This included upright posture, looking down instead of staring or rubbernecking, not throwing one's arms randomly like a child, eating carefully etc.

As far as oral tradition, the explanation goes that your posture & grace is tightly connected with one's emotional energy and the state of mind. In Vajrayana the entire emphasis of mid-to-higher tantras is on one's energy expression, so there they practice extensively to have the right type of "deity pride" including corresponding posture and mood.

In my opinion, this is actually a very important part of the Teaching that deserves more attention. The three separate phenomena we call "body", "emotions" and "subconscious mind" are in fact facets of the same continuum. So the way we carry around our body, move our hands, use our facial muscles is a very direct expression of our awakening (or lack thereof).

Although, for a student, the body practice is mostly about internal benefit of overcoming one's bad emotional habits, for a teacher it becomes a tool of external influence and motivation. The right posture, grace, smile, demeanor are all parts of the rainbow body experience, utilized to arouse in students confidence in their teacher and entice them to imitate and develop similar wholesome qualities of mind.

In the Theravadan tradition, most of the rules governing posture don't come from the suttas, but rather the vinaya. Specifically, they are found in the 75 sekhiyas. The first 26 are almost all about posture.

The reason for these practices is really no different than those related to appropriate posture during seated meditation. And it has a similar set of challenges. In the West, we often set up a dichotomy between the mind and body. No such division exists within Buddhism. Form is just another skandha. Just because Buddhism meditates on the foulness of the body does not mean that it should be ignored or viewed as somehow less important. The body is also a tool that can be used on the way to liberation. A posture maintained ardently, with mindfulness, and clear comprehension will have a comparable impact on cognition. It establishes alertness and vigor while a bodily posture that is not meticulously maintained falls into sloth, heedlessness, and dissipation. An upright body is an upright mind. They are not different things.