One of the most profound things yoga has taught me is that everything in our bodies is connected. For example, if the upper back hurts, it may be due to tight chest muscles. If the low back is in pain, it is often due to tight hamstrings. This was such a huge revelation for me, even though I guess it's pretty obvious. I became a bit more aware of my body, and how certain things I was doing was affecting how I felt. (For example, pay attention to how you sit. I used to always cross the right leg over the left, and this was causing one hip to become really tight.) Anyway, Prasarita Padottanasana D (or wide legged forward fold D) is awesome because not only is it a hamstring stretch that'll help release low back tension, but it also strengthens the arms as well (provided you're actively pulling the upper body down). Give it a try!
Tip for Beginners - if you can't reach your toes, just grab the lowest part of the leg you can. It's more important to have a flat back and broad chest than it is to get your hands down to the feet.
PS- Many more yoga pose diagrams, and the best trick for stepping to the front of the yoga mat from downward dog.