CORE EXERCISES!!! No seriously. Anything that strengthens your abdominal muscles. A weak lower back is the result of weak abdominal muscles, especially the obliques and the lower abdominal muscles. So your back muscles are doing double the work. I found my lower back problems went away about a month after I started working my core. Not only do you get 6 pack abs (it's really 8 by the way) your back will thank you!