I was browsing through the sketch forum on CGTalk and I came across todays sketch which was "fire storm". Anyways, my idea was to do storm (eg the xmen character) but with the ability to control fire! Anyways, I kinda think this wouldn't class as a sketch and I won't have it done today so I wont be able to post it in the sketch forum...so here we go. The first progress pic.
as you can see I'm going for a more...scantily dressed storm as you can see I'm working bottom up! I'm happy with the legs so I'm just moving up onto the thong.
You have a nice dynamic gesture and I like the way the arm looks sketched out. I would look up a little reference for this. Her abdomen looks like it has little wrinkles or folds on it rather than muscles.
I think you need help on your flesh tones, as well. Using brown as your shadow color isn't really working. Since it looks like you are imagining a warm light source, it would probably be better to have cool shadows.
keep it up,
Edit: I just looked at your reference pic, and I think you are copying the pose a little too much. It's ok to do this for practice, but I don't suggest this as a general method of working.