WantFit
Cathlete
I happened to run into this core exercise video in NYTime.com. It gave some value points on how to strength core when you have back problems. I post link here to share.
http://video.nytimes.com/video/2009/06/17/magazine/1194841000095/core-values.html
http://video.nytimes.com/video/2009/06/17/magazine/1194841000095/core-values.html