A very very low refined sugar diet. That means extremely limited amounts of sweets, sodas, white flour, etc. Stick with complex carbs instead of simple sugars. Complex carbs would be veggies and whole grains.
Fruits are good too, but focus more on the veggies.
The right genes. We all can have a six pack, but some women are just genetically predisposed to carry a layer of fat on our abs so that you will never see it, and others can get one with a lot of hard work. A good solid ab routine. Functional movements can really help like hanging leg lifts, decline sit ups, etc. along with a good crunch and oblique routine.