Practice proper breathing technique when working your abs. Always exhale on the exertion. This decreases internal air pressure, allowing you to fully involve the abdominal muscles. No amount of abdominal exercises will reveal well toned muscles if you're padding them with an extra layer of fat. If you're still hanging on to a some extra pounds, this is the area where a few of them can most certainly be found.
Abs are very stubborn.
Other parts of your body, such as your legs, will reveal your training efforts and muscle tone much more quickly, since they are much less prone to storing fat. Eating habits are just as critical to leaning up your midsection. Replace sodium, sugar and preservative laden foods with fresh whole unprocessed foods. Drink as much water as you can all day long to flush away bloating. Also, keep your diet balanced, eating a healthy ratio of carbohydrates, fats and proteins.