Whenever doing core exercises like crunches, plank position, or push-ups or any sort of cardio work out (e.g., kick boxing, zumba, etc.), I always make sure to engage my core. This means pulling your navel towards your spine and visualizing them touching. It strengthens those muscles and helps your midsection look flatter sooner.
If your stomach pokes out while you are working on your core, your muscles will begin to develop that outward pooch.