Josie...

I'd have to agree with Meg about Pilates. Great for strengthening lower back!! Here's something I like to do to get rid of lower back flab...

Buy one of those big inflatable exercise balls (about $10 at Walmart or similar store.) Lie on the ball, tummy down. Keep your head facing down. Raise your right arm straight out in front of you while raising your opposite leg. Now switch...raise your left arm and right let. After only about three on each side...you'll feel it. You can also do this lying on the floor without the ball. I think the ball gives you more support and a better range of motion, though.

Good luck,

Megan

p.s. look into getting Pilates...I think it can work for everybody!