I just learned a new way to tie the Mei Tai for a back carry - I posted here...http://www.thebabywearer.com/forum/v...ic.php?t=36769
Though, honestly, I have not yet put the baby on my back with it yet. I've mostly been carrying her on my front.
Sometimes I tie the straps around her back, sometimes around her bum. It really depends on what she is doing. If she is nursing, I tend to tie it around her back more because she is down lower on my torso. If she is sleeping I tie it lower because I tend to carry her higher, if that makes sense? I've found that the carrier provides enough head support because mine is curved up at the top - I think the Baby Hawks are like that too.