Some people can't get a six-pack out whatever they do. Some people can only do it if they change their diet, e.g. go oon the Atkins, become a vegetarian, give up dairy, blah blah blah.
You can probably get more visible muscle if you add more variety to your abdominal exercises. Still do them until you're sick, but throw in lots of different types. These are some that I do:
- Regular sit ups. Arms right behind the head, head back, body straight (does the middle and bottom).
- Crunches. Sit ups with a curling upward motion, does the top.
- Rotatating crunches. Get in the situp position and bring one foot back to your ass so your knee is up in the air. Put your hands behind your head. No do a crunch, bringing the opposite elbow to the opposite knee.
- Situps with elevated knees. Get into the situp position and lift your knees until your shins are parallel to the floor. Now do either regular situps or crunches. Don't lower your legs between sets for extra pain.
- Stomach wheel. Like a little bar with a wheel or two on it. Kneel on the floor holding this, and roll out, then pull your weight back in. These are good, although they did seem to inflame my sciatica, when I had it.
- Leg raises. Lie flat on your back. Lock your legs and lift them 1 inch off the floor, hold for ten seconds, then move to 45 degrees, hold for 10 seconds, then move to 90 degrees (without taking your tailbone off the floor), hold for 10, move back to 45, 10 seconds, back to 1 inch, 10 seconds. Repeat this 4-6 times. (This is a good one for doing the whole length of the abs.)
- The fish. This is an Akikido exercise. Lie on the floor wearing some shorts or something that will slide around a bit. Lift your torso and legs just off the floor so only your backside is on the floor. Now do an impersonation of a fish out of water, wiggling your butt from side to side. Keep going for a minute or two. You will look like an idiot, but it is great for the lateral abdominals.
There are plenty of others, but these should help. Most importantly, you should feel the effects without having to do hundreds of each.