Abdominal crunches only focus on your rectus abdominus (6-pack muscle). In addition to performing crunching exercises, you should focus on strengthening your core muscles. These muscles surround your rectus abdominus and connect to your back musculature. To truly tone your stomach muscles, you need to address more than your 6-pack.
An example of a good core strengthening exercise is to lie on your back and lift your knees to a 90/90 position. Lower one leg to tap your foot on the surface then bring the leg
back up. Alternate lowering each leg and tapping each foot on the surface.
Perform this exercise for 10 reps. Progress to 2 sets of 10, then 3 sets
Check out the video below for a demonstration of this exercise by Dr. Amanda Macht
Lumbar Stabilization Exercise- Table Top Taps