Because someone mentioned sacroiliac issues a while back, I thought I'd link some resources that I've found helpful. I discovered the hard way that it's easy for the SI joint to unlock and move about more than it should, so learning to stabilize it has been an important aspect of my practice lately.
These two little videos were almost miraculously helpful:
Lillah Schwartz: Stabilize the Sacrum with Yoga -- Part IStabilize the Sacrum with Yoga -- Part 2Yoga Journal: Practice Tips for the Sacroiliac Joints -- this is oriented towards teachers, but I found the "Move it with care" section especially useful.
And the one thing that turns out to be critical for protecting my SI joint and my lower back in general: engaging uddiyana bandha.