Training the ab muscles is greatly beneficial, not least for health reasons, but if your goal is visible washboard abs you're going to have to do something to burn off bodyfat too.
This way, you can dial it in nice and slow -- aim for losing a pound of bodyfat per week -- without falling prey to the classic yo-yo dieting fallacies.
That's good and dandy, but in order to get a slim waist, you have to burn off the bodyfat covering the muscles.