While ab and core exercises strengthen your stomach muscles, they do not burn fat (ie - you can't spot reduce fat). You must continue to work out, eat a good diet and over time it should slowly decrease.
On a video where Ryan Reynolds trainer was talking about getting Ryan into shape for the Green Lantern, he said "while working out is important it's 99% diet." What he mean was that to get lean and cut the real difference maker is diet. The other big key is reasonable time.
During my first lifestyle change I lost 30 lbs. in 7 months so basically 1 lbs a week. And then during a last 3 months I started to refocus on fitness, saw the Ryan Reynolds story and started to focus on my diet. I got a Calorie tracking app, changed eating habits, added some new interval cardio routines and the result is almost 10 lbs in 3 months and my abs are popping again.
While I have worked out abs consistently over the past year, it was only when I adjusted my diet and focused on interval cardio activities that I started leaning down and getting the look I wanted.
As for your food question, here is a simple rule. The more natural the food the better. Some key foods I enjoy - chicken breast, fage 2% greek yogurt, oatmeal (not flavored), raw almonds, peanut butter, carrots and muscle milk shakes. While I am still cutting down I aim for 1500 - 1700 calories a day and more on big cardio days. The other secret is... if you don't want to eat a 150 cookie or other high calorie item - Don't Buy It. You can't eat it if it's not in your house.
I hope this helps, now I am off to the gym.