Pilates. Pilates focuses on strengthening the core muscles, which are the abdominals. You should always any regimen under the guidance of an experienced trainer to avoid the risk of injuries. Weight training is also a good option. I would recommend both pilates and weight training and i can say from personal experience that they are effective.
Crunches. Doing regular crunches and crunches to each side of your body repetitively every day will tone your abs. Do this for at least five minutes every day and you will notice a difference with time. When you do the crunches, however, make sure that you tense your abs; make sure that your body moves because you are tensing your abs. That is the true way to do a crunch.