People who have the goal of enhancing their mid-section are often looking for a new stomach exercise to perform. They keep searching for the latest and greatest workout that will give them the definition they are hoping for. Unfortunately the best stomach exercise to perform isn't a stomach exercise at all!
To match the goals of having a lean and strong midsection, your best bet is to lift heavy weights in the gym for the rest of your body and then hit up the cardio equipment for some high intensity cardio training.
To start, the heavy weight lifting will help call all your core muscles into play, strengthening them and building the muscle size. Be sure that you are fully challenging yourself so that you are fatiguing within 6 to 10 reps.
Then the high intensity cardio portion of your workout will be intended to strip away any fat that is covering the midsection, revealing these strong new muscles. This point is really key here. If you want the definition, you've got to put in the cardio work - or be incredibly strict on your diet. Most people will choose the former and part of the latter since this is a slightly easier combination. Not too many people want to exist solely on chicken breasts, egg whites and vegetables. Keeping the cardio in the program helps you slightly expand your dietary choices.
So definitely don't discount sit-ups from your program because they do round out a good lifting session nicely but when it comes to really noticing results in this area of the body, focus more on the other two aspects of your training as that's where the real secret to great abs lay.