I use the tennis ball method of massaging the SI joint area. For example, go to http://kelziebeebe.blogspot.ca/2011/06/self-massage-with-balls.html and search for the phrase “Butt and Back of the Thigh” without quotes. I find that the tennis ball gets soft very quickly, and I don’t like the variation of hardness as the tennis ball ages and gets replaced. Is there a hard-ball version of the tennis ball for physiotherapy purposes? It should be textured on the surface so that it doesn’t slip as one rolls around on it. The closest I found was a hard BASEBALL, but only texturing is the threaded seams.
From surfing the web, I ran across the idea of a lacrosse ball ( http://en.wikipedia.org/wiki/Lacrosse_ball ) but it might be a bit small (or my cheeks are too pudgy).