A lot of standard yoga poses really strengthen the back. Are you doing any yoga?
I do a fitness class where he has us lay on the gym floor chest down an we do this Superman pose. First we raise our chest/shoulders up off the floor and extend our arms like Superman. Then he has us lift our legs up off the floor which you can really feel in your butt muscles and hamstrings. Then we do the full Superman combining both chest and lower body & this time leaning to the right then to the left. It is fun, and for a couple days after doing it I get sore. I suspect it also strengthens the back.