Core training is very important---actually it is the foundation for all other fitness training. "Six-pack" abdominals are a small part of the total core area. Formally called the rectus abdominis, this is the part of the abs that runs down the front of the abdomen. One of the fringe benefits of fitness is that you look and feel better. If you do this superset 3 times per week, 3 sets/15-25 repetitions, you will be on your way to building your "six-pack." There's one catch---you MUST eat properly or you will only have a layer of fat hiding your muscular abs! So, here's the superset:
Abdominal ball crunches followed immediately by Captain's Chair Knee-ups or Straight Leg Raises ----Rest 1 minute and repeat the superset. The Captain's Chair is the apparatus where you support the weight of your body with your forearms (allowing your legs to hang) and then you lift your knees/straight legs up towards your chest.
Once 3 sets becomes easy, add weights to further challenge your muscles.
Oh yeah, don't forget about the other areas of your core like the obliques, low back and pelvic area.