I always print square from 6x6 negatives and never crop anymore than about 5% [as my viewfinder is approx 95% and occasionally I find something on the neg that I hadnt intended]
To answer your question; what's the last thing you do before you squeeze the shutter?
I cant really explain it but I just look for a sort of balance, I know it when I get it.
Rather than focusing on the theory of the square composition I check out different photographers that use this format and see what I think works and what I think doesn't. I regulary go to www.hasselblad.se and look at their gallery, both their masters and forum magazine which I subscribe to. I also buy books by square-eyed photogrphers.
I agree with you, people love this format. Personally I cant understand why someone like Michael would buy a square format and almost never print square. To me there is a mystery about it, I even find it difficult to use 35mm [except xpan 65mmx24mm] as I'm conditioned to the square.