This exercise sounds easy but can be difficult to do well. It uses different muscles to those normally used and can be tough to get right at first. Essentially in involves contracting your stomach muscles to pull your belly button back towards your spine, compressing your stomach as you do so. You can either try this lying on your back or in a kneeling position. To start relax your body. Then slowly contract your stomach as if you are pulling your belly button backwards.
Hold this for ten seconds then release slowly. Once you find ten seconds easy repeat it for longer. Building this up over time will lead to a strengthening and tightening of the transverse abdominals, one of the key interior muscle groups.