What is the best way to strengthen my back?

Strong ab's helps. Strong abdominal muscles assist the back in lifting things and in holding steady the top half of the body. The spine endures much stress if it alone has to keep the middle of one's body in position. Strong abdominal muscles act as a flexible, but stiff-when-needed, muscular cylinder that holds one's organs inwards, and firmly connects the bottom of the rib cage to the top of the pelvic/hip bones.
Core strengthening. There are many isometric exercises that are done in yoga and pilates that strengthen core muscles like plank.