Side to Side
Strengthening your core is essential in athleticism. Knee circles will strengthen your core, obliques, increase shoulder flexibility, strengthen and stretch your lower back and allow you to become more flexible. Your timing will also be improved.
Put your knees on the exercise ball with your hands on the floor. Keep your knees together. Your upper legs must remain vertical. Make sure to have spread your arms wide and straight so that you have a wide base. Spreading your fingers will also provide more balance. Touch your chin to your chest.
Roll your knees to the left. Try to touch your left hip on the ball. Now switch sides. Try to make your right hip touch the ball.
Your hands are essential to your balance.
Focus on keeping your legs close together and perpendicular. To help this, get a light hand towel or washcloth to hold with your knees. Hold this while you do your knee circles.

Comments
Re: Side to Side
Like most exercises I believe the best is to do as many as you can while maintaining good form. Consider increasing the speed of the movement if you can with good form.
Re: Side to Side
how many reps