I have some questions on the posture. The background is that I my back has been a bit messed up the last four years. The last year or so I've developed a semi-regular yoga work out which helps and I´ve met a back specialist who has also given me some work outs which I do every morning so now I can sit as long as I don't do it too much. However Some days I can't sit at all and my overall relationship to posture and sitting is one of uncertainty and skepticism. I've sat in so many strange ways so as to avoid back pain over the years and I have a really hard time knowing when my posture is correct.
I can get in either a decent or awful full lotus depending on which foot i begin with, though it isn't very comfortable yet, I wouldn't say its very uncomfortable either. I think it does improve my posture though but I could be mistaken.
I sit regularly with a christian community in my area which has biweekly "christian" zazen and they sit in seiza on meditation stools which also helps with my posture, only I wouldn't know where to get a meditation stool around here.
Seiza without a stool is also good for posture but my feet hurt quite a bit when I get out of it. Perhaps I mess it up somehow.
I've also sat a bit in the burmese position on a cushion which is comfortable but leaves me clueless as to my posture compared to the above. Perhaps part of my problem is the cushion being too soft.
I've also started doing walking meditation so I can meditate even when my back is messed up - which is great.
Right now I'm thinking I'll just include some extra stretching into my regular exercise so as to make my lotus more stable but then I'm not very sensible when it comes to my back.
All the best,