Of course, don't give up on crunches. Incorporate moves that hit the other two angles and your abs will be much leaner. Even if you've got a fantastic workout, you still need to vary your moves every few weeks. The longer you stick with an exercise program, the more efficient your body becomes at doing it. You end up using less energy and burning fewer calories, and you won't get optimal results.
Research shows muscles can adapt in as few as four to five workouts.
Just tweak your moves. For example, instead of placing your hands by your ears when you do crunches, try extending your arms in front of you. Or do oblique twists on an incline bench rather than the floor. Exercising while holding a weighted ball or dumbbell is another way to switch things around. So many women think they have to knock off 100 crunches in a row to get flat abs.