I read this the other day and I thought it might be helpful to share.
Ask Denise Austin
Why Aren't My Abs Toned?
Q: I do crunches religiously, but I don't seem to get more toned in my abdominal region. What's my problem?
A: You may need to spend less time doing crunches and more time on the treadmill, stair climber, or stationary bike. Contrary to popular belief, abdominal tone doesn't come from doing a million crunches. In fact, you may already have abs of steel — they simply may be covered by a layer of insulation. So, instead of doing 10 or 15 minutes of crunches, try doing three to five minutes of exercises targeting different abdominal muscles as well as your lower back; focus on using good form and really tightening those muscles. Then spend the extra five to seven minutes burning fat with a cardio activity such as running or cycling.