Exercise that strengthens your core muscles works best.   Working with a personal trainer, yoga, or, for me, Pilates, are 3 ways to strengthen your core.

I have only personal and anecdotal evidence, but I've had great luck with Yoga building core strength, the back and muscles that support the back. I found an article on yoga.com regarding back pain specifically.  A word of caution, yoga is also a great way to injure your back.  I'd suggest going with a small studio and smaller classes where you'll get one-on-one attention.

I too have dealt with lower back pain and the best solution to deal with it is to work with a personal trainer that will put an alternative fitness program together that focuses on the health of your core.  Through a variety of both stretching and strength exercises, I have improved my core's strength and have significantly reduced my exposure to back pain.  It truly works!