i had the same question a week ago.
i got a nice direction on this from a memeber over here (Ron).
and its accually working!!! u gotta keep a ratio of 50/20/30 (Proteins/Carbs/Fats), avoding any unhealthy fats such as any kindo sweets...
first u gotta know how many calories do u haveta eat per day. then use the 50/20/30 ratio for ur diet.
trust me its pretty hard to keep a diet like this, but well im keeping my diet close to those number and it works very well!!
BUT, this diet wont give u any muscle gains elsewhere. if u r only working on ur ABs then i dont even know if this would work. but anywayz u just gotta keep working hard on ur other muscles along with ur ABs.