Also, Noah had asked the other day about how to sit to relieve back pain. Here's one idea:
Sitting with belt support. Use a yoga belt (or any belt which is big enough to go around you from lumbar to easy-seated-knees). Snug the belt up til you feel like your knees and back are mutually supported and see how it feels for your meditation. You may also notice that I am sitting on a wedge. This tilts the pelvis forward and relieves back strain.