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You need low enough body fat and big enough muscles combined.
Neither will work by itself.
Crunches with weights. Weight-Vest or weight plate or dumbbell works fine.
Reverse Crunches is the best exercise, though. No injury or pain, only gain. Weights can be attached to legs and if you have access to an elastic band, use it.
If your gym have ab-machines, use them. Keep a lot of variation in your ab exercises to keep growing. Don't over train, you won't grow at all.
Well, that's about it. Keep your diet healthy and don't forget to eat a lot to gain a lot.
Train a lot, but not too much. Three times a week, non-consecutive days.
Great way to check your growth is by taking pictures. If you have a webcam, i suggest you take weekly pictures and compare gains. See if you're doing too much or too little.
Source(s):
MMA/BJJ
If your abs need rest, train them only twice a week, then after that once a week. Keep it up a week, or two, then do abs three times a week again.