According to Shape magazine, do not do crunches to get six-pack abs. Crunches are less effective than other exercises for this purpose. Instead look for exercises that target the whole core.
Lou Schuler, co-author of "The New Rules of Lifting for Abs," says that crunches are not effective for all abdominal muscles; they only work the front and sides. Schuler maintains that there is much more to toning and defining abdominal muscles. Other parts of the core must be targeted, too. She suggests doing a series of core stabilization exercises that work more muscles to burn more calories than crunches. Three of these exercises are the side plank, the walkout from pushup position and the alligator drag.