There are many exercises and movements you can do to tone stomach muscles. Women and men generally undergo very similar training to tone the abdominal muscles.
As with all exercise, you should continue to the point until you can no longer do the action, ie to the point of muscle failure. This is what helps the muscles to grow.
Girls, here are some of the most effective (and enjoyable!) abdominal exercises:
Exercise Ball Crunches
You need a regular exercise ball for this one.
1) Rest the ball under your mid to lower back and lie face up.
2) Arms crossed over chest or placed behind head.
3) Tighten your abs and lift your torso off the ball.
4) Lower back down again and feel a stretch in the abs.
5) Repeat up to 16 times.
Reverse Abdominal Crunch
1) Lie on your back on an exercise/yoga mat with arms by your sides, feet up and thighs on a 90 degree angle to the floor.
2) Bring your knees to your chest while tucking your chin to your chest at the same time. Use your lower abs to do this.
3) Return slowly to starting position, repeat.
Sit-Ups
1) Lie on back with feet flat on floor, knees bent.
2) With hands behind head, bring yourself forward and touch elbows to knees.
3) Keep entire stomach area contracted throughout.
4) Lower body back to starting position and repeat until you can no longer perform a sit-up