(copied from THIS link )
Imagine you have a friend small enough to fit in your purse or briefcase who will give you nightly massages, easing stress, fatigue, and muscle tension all for a onetime cost of about $3. Think yellow. Think tennis balls.
The procedure is simple: place a tennis ball on a part of the body that is sore or tight and rest your weight on it. The pressure softens tight muscles and increases circulation. Dancers have long used tennis ball self-massage to work out their kinked-up muscles, and body workers frequently prescribe the practice for clients healing from chronic injuries. Asana practitioners searching for ways to increase flexibility and comfort in their bodies are beginning to get on the ball, too.
Self-massage with tennis balls is greatly enhanced by stretching, which further increases circulation and reeducates the muscles to rest at a longer length. Combining asana with self-massage is particularly useful for those of us who have muscles that for various reasons—injury, overuse, or years of inactivity—resist stretching. Here are some tips for using tennis ball therapy to unlock the gluteals, lengthen the hamstrings and adductors, and restore balance to the muscles along the spine.
We’ll start with a simple technique so you can get a feel for how this works before going on to more complex applications. Lie in savasana and place the tennis ball in the center of the fleshiest part of the left buttock. The pressure may be painful, but if it feels like good pain and you can relax in it, stay there for a few breaths. (If you find you are contracting muscles in defense against the pain, move the tennis ball around to find a spot that is less painful, or practice on a padded surface, which will reduce the pressure.) Relax and breathe. Imagine the breath circulating around the pressure and visualize the muscle softening over the ball. After a minute or so, remove the ball. Before repeating the process on the right side, notice how the left buttock feels flatter than the right and softens more easily into the floor.
(click HERE to read the rest of the article!)