My last drawing today was this back study from a posespace.com photo. Luckily there wasn't too much going on other than some suggestion of scapulas (scapulae?), the spine, the sacral triangle, and a couple hips. I've seen well-developed guy's backs that would drive you crazy with all the lights and darks.
I'm using vine charcoal for the basic drawing with a chamois for erasing, and then compressed charcoal at the end to emphasize the blacks, and a hard eraser to take out some highlights. I'm still struggling with relative values. I'd also really like to find a better paper.