Just after I finished teacher training in 2008, I pulled my sacro-illiac joint out of alignment. Ever since that happened, my SI joint has been problematic. It's not necessarily painful, but I do need to take extra care of that area. Now that I've been incorporating more yin into my practice, my SI has stopped giving me grief, and in in general, my lower back and hips feel really good! This initially was surprising to me, because I didn't expect that doing a different style of yoga would effect my flexibility and general well-being so much.
Yin yoga is a beautiful practice that involves holding poses for several minutes at a time to gently stress the connective tissue (ligaments and fascia) to create and maintain range of motion in the joints, especially the back and hips. It is a wonderful complement to "yang" excercise (vinyasa or hatha yoga, weight training, cardio, etc.) that strengthen muscles to stabilize the joints. It also works under the same principles of acupuncture by stimulating various energy lines in the body.
Here's a short Yin sequence that I've found helpful for my SI issues, and may also be helpful for general low-back pain. Obviously, if you're in pain, I recommend seeing a doctor, and following his or her advice. Can I say it again? YOGA TEACHERS ARE NOT DOCTORS. I can't diagnose you and I can't cure you, and I certainly can't do it over the internet. So, while this practice has been extremely helpful to me, please be wise and trust your body's knowledge. If something hurts or otherwise doesn't feel right, please stop. It is straight-up stupid to hurt yourself doing yoga.
Thanks for bearing with me through that disclaimer. Onto the practice!
In this sequence, we do some forward bending, which makes space along the back side of the spine. then we take it in the other direction, backbending, which culminates in Saddle pose (aka supta virasana). This pose is my all-time favorite and is magic for putting SI back in alignment. Even if your SI is perfect, this is a great practice for the back, because it flexes and extends the spine equally.
Hold each of these for up to five minutes. If you're new to this kind of practice, start with holding no more than two minutes.
Child's Pose, knees wide arms overhead
Sleeping Swan (4 min) and Swan (1 min) -- This is awesome for SI issues. It counteracts any chronic tightness in the piriformis (outer hip) which can pull the sacrum out of alignment. Coming up to high swan is the introduction to backward bending in this practice, so be gentle here.
Locust hold for 5 breaths, 2x, rest in between
½ Saddle (Saddle with one leg straight)