One of the most common problems we see at the studio is back pain. There are all kinds of causes. Sometimes it's an old sports injury. Sometimes you just moved a box the wrong way. It can even be something as simple as sitting at a desk all day long. Whether the origins are some acute injury or long-term stress, there are a few simple poses that can help (for acute injuries in particular, it's good to check with your doctor for more detailed instruction).
Something to keep in mind throughout all your yoga -- breathe deeply, and match your moving with your breathing. If you can breathe slowly, easily, with attention -- your moving can be the same. When handling injuries, this is especially important. The muscles around an injury, and even radiating throughout your body, will tighten to defend and compensate. This may protect against further injury, but can prolong or even prevent complete healing. It takes time and patience to let these muscles release. In the case of back pain, giving time for your muscles to release can allow your vertebrae and discs to align properly, and healing to proceed more easily. If something hurts, back off. Just breathe, move slowly, and pay attention.