This is one of the best exercises to strengthen the hip and hamstring.
Lie down into supine position; put your legs on the ball. Knees bent, your feet on the ball, support yourself on the ground with straight arms. Lift your hip high, hold it for a few seconds then slowly lower it. Pay attention that when lifting your hip, your back and torso should be straight all the way. You can make the exercise more difficult by executing the one legged version.