Answer:
I use castor oil packs. It sounds odd, but it is effective. Just soak a washcloth with castor oil (you can get at a pharmacy), and heat it in the microwave as hot as you can stand it. Lay flat on your stomach with the castor oil cloth over the sacral area, and let i stay there until it gets cold. I have had to repeat this process 3 or 4 times, but it has helped me. It's very uncomfortable, and I hope this works for you!
I have the same problem, but I'm 15. Lay on a hard flat surface is the easiest, on your stomach or back. If that doesn't work, lay on your stomach with your arms to your side. This is going to sound wierd, but push your butt and only your butt up. You will probably hear you back pop, and thst should do it. Or stand flat and stretch up as far as you can. Another method is standing straight, then arching forward, so it stretches your back out.