Not sure what exercises to do on the ball for your back but cat and dog yoga poses are great for stretching out your back and also have your DH rub a tennis ball on your back - it's wonderful.
Right now I just like sitting on it when I'm watching TV instead of the couch...it seems to put my hips in a better position and relieves some of the pressure on my lower back (she is sitting on my sciatic nerve).
Just sitting on it instead of a regular chair makes my back hurt less. Try to tuck your pelvis up as much as possible while sitting and roll around or bounce a bit - helps keep things loose.
Re: Dumb Question: How to you use the exercise ball?
Breastfeeding Counselor with Breastfeeding USA
Babywearing Guide ** Newborn Carriers
Cloth Diaper Guide
Safe Bed Sharing Info