Although crunches do aide in building a better core, they are not the only tool that should be used! The Superman Exercise (seen above) engages the abdominal muscles and works the back. Despite seeming counterintuitive, it is crucial to build back strength in order to have a more solid core.
In order to properly perform the Superman Exercise, lay face down on a matt. While engaging the core muscles, glutes and hamstrings, lift the arms and legs off of the ground. Hold this "flying" pose for 10 seconds then release back to the ground. Repeat this movement five times in conjunction with an abdominal strength routine.
((Photo Source: http://www.health.com/health/gallery/0,,20664616_17,00.html))