First off: congratulations! It is good that you see progress in your own drawings, that's why it also good to date all of the sketches you decide to keep (and as you don't know which ones those will be, date tham all).
Now, I'm not the expert (as you can see from looking at my blog, you'll find a link in one of my past remarks in the forum), but here is my 10 cent's worth of critique.
It is a great sketch! I see you are using a lot of shading and that the direction of your lines also suites the orientation of the planes very well. You could perhaps work on a neater style of line for the shading. What confused me a little with your sketch is the lack of a line to help me understand the twist of the abdomen better. You have strong lines on the legs and pelvis. But the abdomen is covered in a sparkling/blinding light and lacks that one little line ... perhaps even the spine (or the lumbar valley it is contained in) could be slightly hinted on. Hair could do with a very slight line to cover the difference of texture (skin vs. hair). I was also a bit confused by the eyelashes, the darkness seems to be hinting on a plane/orientation change. I love the model's right hand, but I'm a sucker for hands.
Anyhow, this is a good sketch. Don't bother too much about it for a few days. Put it away, after dating it and take a look at it again next week. In the meantime, go for it, looking forward to more of your sketches.