As with any other aspect of your fitness routine, you want your stomach program to be as all-inclusive as possible.
Just like the other muscle groups, your core muscles need to be worked in a fashion so that you are both challenging them and then also letting them recover in order to improve. Far too many individuals perform exercises on a daily basis hoping to achieve a strong midsection. This either signifies that they are likely on the road to overtraining or else not using correct form.
When performing your stomach program, you want to include a variety of exercises and keep changing them on a regular basis. Your abdominal muscles in particular have amazing adaptability and will learn to deal with anything you throw at them quite quickly. Once they have adapted, this means that if you no longer keep changing things around, you are going to see stagnating results. For this reason, try and change the actual exercises you are performing at least once every month, preferably every two weeks if you can remember. It doesn't necessarily always have to be a completely different exercise but something about it must change, either the way you are laying while doing it (for instance rather than always crunching up, try crunching to the side), the resistance type you are using (try using a cable pulley rather than a weight) or the base of support you are on (move to an exercise ball rather than the floor). All of these things will shock the ab muscles and get them responding again.
Also, contrary to popular belief, doing hundreds of reps at a time is not smart either. If you are able to do this many reps, you should rather just add some weight to the exercise to make it challenging. All you are doing by performing countless reps is burning calories, and if burning calories was your sole goal, you'd be much better off on the treadmill or the squat machine (the leg muscles are larger hence will burn more fat when worked).
So have a good look at your exercise routine for this part of the body and ensure you aren't making any major mistakes.