What kind of ab work is in your back exercise routine? I hope you include challenges for your oblique muscles - the internal obliques and the external obliques. I say this because I know from personal experience that when you're busy (and weak in these muscles), it's easy to gloss over them unless you have a taskmaster to remind you.
Allow me to be that taskmaster. Along with crunches, which, by the way, some say are of questionable value in the care of the back, a core strengthening routine needs to include rotational and tilting moves. (Check out About.com's Pilate's site for some great alternatives to the crunch.) The obliques play important roles in trunk support and shouldn't be ignored.
If you're a beginning back exercises, or if you're still contending with pain and limited mobility from an injury, check out this very basic oblique abdominal crunch.