Tuck in your tummy whenever you walk. Get that proper gait. And the best way for that is to tuck in your tummy and inflate your chest. Do not let your tummy hang above your belt line like some unruly layer of flesh. Bring it under the belt.
Each time you tuck in your tummy, you will feel the pressure on the muscles of your stomach. This tightening and loosening of these muscles is even better than stomach crunches.