How do I go about it? He lies down on his stomach and you straddle his bum. Place your palms flat on his bum and massage in circular movements, getting bigger as you go along. Increase the pressure gradually. Next, place your hands on his right buttock, and using your thumbs, create a roll of skin that you push up to the coccyx (tailbone). Alternate between the left and right buttocks.
Why is it so good? Because his derrière is a highly erotic area, just like ours! Also, the stimulation spots that control the prostate and testosterone production in men are located around the coccyx, near to the gluteal cleft (i.e. the crack).
A good time to try it: When you have enough confidence in your partner to allow him to get to grips with your bum in the light. One to avoid the first time if you're a bit insecure about your bum!
An extra idea: Vary the tools you use! Rather than your hands, place your forearms on his bum and make circular movements from the outside inwards. Larger movements give more intense pressure.
Note: While light caresses using your fingertips are enough to stimulate the thighs, kneading movements and vigorous massage are more suitable for the bum, which is more muscular.