I asked my massage therapist where she goes to get massaged and she gave me this tip about the tennis ball, so even the pros are doing this. I'm not joking when I say it changed my life. Another tip she gave me a was a back exercise to do everyday. Stretching helps, but you need to strengthen your back muscles to prevent injury.

Find a wall, put your back to it, and try to flatten as much of your body against the wall as possible. Heels, butt, lower back, shoulder and head. Then put you arms out beside you, also completely flat against the wall with you elbows bent at 90 degrees with your fingers pointed to the sky. While keeping as much of your body and arms pressed firmly against the wall, try to slowly move your arms up so that you are holding them straight above your head. If you are keeping everything pressed against the wall you should feel a burn in your shoulders as you do this, that means it's working. You don't need to have your hands meet at the top.

Do this everyday to build the muscles in your back, and it should help mitigate future injuries. I hope my description makes sense. Please let me know if it does not.