Some of the best exercises for building abdominal muscles are bicycle crunch, roman chair, crunches, vertical leg crunch and reverse crunches. However, you will also have to work out the rest of your body muscles for these to be effective. You can get more tips on http://www.mayoclinic.com/health/core-strength/SM00047&slide=3.
Try doing the plank every day. Its where you put your elbows on the ground beneath your shoulders and get in pushup form. Then try to hold it for as long as you can each day and hopefully it will go longer and longer.
Bicycle crunches are probably the best, however, I have found that the ab wheel, $8 or so at Wal mart, is the most effective. You will need to build upper body strength before you can use it properly unless you buy a spring loaded one which will help so you don't have to use as much strength. They are more costly though. Once you have built ab muscles they will not show until you diet correctly and do a lot of cardio. Less than 3% of the nation has visible abs.
The big thing is just doing it. Everything works. Certainly at some level you plateau you need to mix it up. But just hitting crunches is going to build strength. The idea that crunches "have been proven not to work" is simply not true and they is not a single study out there that supports that contention.