In the poses where we lie down on the mat, our teacher always reminds us to press our spines into the mat, and that there should not be a gap; the spine should be flat and not come up off of the mat. I struggle with that because of my rear end. If my rear end were not so rounded, it wouldn't be a problem. I do the best I can, but really! So my question is: How does one do that with a body type like mine?