To get your abs thicker it's all about training heavy on them, same as you would trying to make another muscle grow. Just be carefull, the spinal load can be dangerous if you aren't already strong in that area. If I were to recommend one exercise, I would say roman sit-ups are better than anything else for thickening up the front of your abs, and also for receding your linea alba (center line). Also, know when to stop thickening. You can go too far and they will start to stick out more than you want. Once you get them to where you want, decrease the training load to almost nothing. I haven't done an ab workout in 4 months. I just flex them for stabilization in the rest of my exercises and that is more than enough at this point.