Crunches are the classic ab exercise (although planks and push-ups have their fans too). To really target your abs, though, it's important to use good form.
Photo by the North Carolina National Guard.
A guide from Fitness magazine gives the top five form fixes, starting with the difference between a crunch and a sit-up (the sit-up goes higher and works your hip flexors more than your abs). Other key points are slowing down on both the upward and downward motions, and breathing out as you contract -- which will help activate more of your abdominal muscles.
Check out the details on the fixes at the link below.