The truth you must come to accept if you do want to get results with your six-pack ab training is...
Here are the myth and truth about six-pack abs to make yourself aware of. A very common ab myth is that the best way to obtain them is through hundreds of crunches a day.
This is incorrect because of the fact that crunches actually burn very few calories per minute. When you do them, they will work to strengthen the abdominals but they won't do a lot to help shed the layer of fat covering the abs. The truth you must come to accept if you do want to get results with your six-pack ab training is simply based on the way you eat and the way you do aerobic exercises.
Indeed, in order to reduce your fat mass a low carb and low fat diet is recommended. Unless you're starting at a very low body fat level already, you must be patient with the process as it will take some time.
Conclusion: If you want to get six pack abs fast, try to lose fat first. Losing the fat is done through diet and aerobic exercises, not abdominal crunches (or sit ups).
Tips from an "abs pro" in this video: