I think it helps to just draw the major shapes and how things overlap when doing line work and not necessarily show lines of all the little muscles in this case. I also noticed the little hook you put at the end of the arm where it attaches to the back shoulder blade. If you follow the shadow of the muscle, it doesn't curve down like that. It curves slightly the other way. I would also remove those muscle lines right next to it as it throws off the look of the back. The same thing with the line at the bottom of the waste. It throws off the anatomy. If you follow the shadow it connects to the edge, close to the bottom of where the rear starts to help show that the rear pops out slightly more then the bottom of the back. The dimples right above the rear of her back should point to more of the center of where her butt crack starts, like a V shape.
It's important to learn anatomy even if you use pictures as reference, so you know muscle placement and how they bend and move. That way when it's not shown clearly in your reference you can make liberties and make your own lines to show muscle placement, as well as modify the image to make it your own, which is always good. Make sure when using a reference you aren't just copying the image mindlessly. Study it, pay attention to the placement of objects and more importantly understand why it's placed that way. The more you understand the better your results. The same thing goes with lighting, composition, color and etc. Even when I'm not drawing, I sometimes just look at people when I'm at work or other places and try to understand the placement of there anatomy depending on there body position. It helps build up your visual library and spot things that don't look right to you when sketching. Sometimes I space out when people are talking to me because I'm looking at their face trying to figure out how the lighting affects the values and placement of shadows. Which probably makes me look weird to them.