Regular crunches (short sharp movements to compress your abdominal muscles) are very good for working the front part of your stomach, but do little for transverse abdominals deeper inside the midsection. While better than sit-ups, they still put strain onto your neck and back. This first exercise addresses that, working your transverse muscles more fully with less likelihood of neck or back strain.
To start, lie on your front or kneel. You may wish to try both to see which suits you to 'feel the burn' of the exercise more fully. Relax as much as you can, and try to use just the lower abdominal muscles to pull your belly button inwards toward your spine. Maintain this for at least ten seconds. If when you do this holding it for ten seconds feels too easy then build this up and hold for a slightly longer period. Then let the contraction go. Repeat this for full sets and over a relatively short time you'll start to feel the benefits.