This is to echo what was said by Chet and Jundo the wonderful thread about shadows.
Let me again and again repeat a few words of caution: sitting with too much tension and will to be upright is really muscle building. In my modest opinion, the right thing does itself, which means that one grows a zazen like a tree or a plant would grow, slowly expanding, naturally going up. It is much better to start with a fairly relaxed sitting and allow the body-mind to go up rather than freeze the whole thing into a military pose that eventually collapses. Most of the agression that I met in the Zen world came from people sitting like in a soldier parade, chin in and rigid back. This is a tendency that Mike is aware of as I am too. People displaying anger should watch the way they sit, as they get tense they just get the whole picture far worse, thinking that they get it right, do the right thing and so forth they are being a pain to themselves and the world. In other words, yes sitting like a concrete pole is not the gate of ease and joy,it is hell's gate. For twenty years I sat in that style and developped a highly agressive style. Nowadays, I feel sometimes sorry for what I have done and what I have told people. But I also feel very happy to have met this possibility to sit at ease, in a painless and open way, to have really understood how rigid and deluded I am. Investigate your sitting and notice how much you overdo things. That's why Alexander technique is important for me as an eye opener and not a new gimmick. And they are many other ways to look at it. Yoga is great ( but still too much doing...) And the chances are that you are still overdoing it. The level of resistance to what is happening in our life, our resistance Kannon as reality, or our over-enthusiasm to practice everything in the right way...Overdoing. Let's look at it. Please, this is important.
Correcting somebody's sitting is really the wrong direction ( as it always done in Zendo). To correct one's sitting is the wrong direction ( due to false sensory appreciation ). So what to do? Read and undesrtand Dogen, look at Ryokan playing and sleeping, look at small children or animals and don't worry. Worry no more. The moonlight floods the room, flowers and women are sweet, mud is beautiful too. Worry no more.