Hey, Sarah, as a fellow AE, I have to say that your stuff is looking pretty good, but your hand studies are absolutely top notch. They make everything else pale in comparison. One really effective technique that I've been recommending is to draw something from reference, then draw it again without the reference and compare the three. I think that if you start incorporating more of the "real" in your work, it will really take-off.

Cheers!
Tim Hibbetts