One thing I generally find helpful is after you finish sketching out the general shapes of the bodies is to go over all the outlines with a pencil or if you're feeling fancy some kind of fineliner or pen which helps smooth out all the lines. Try to go over each line with one line, rather than making lots of short little lines as in sketches.
If you're having trouble drawing based off of things that are already well defined, just take a note of the kind of clothes they wear and body types they have and pretend you're drawing someone completely new, just with the same clothes and hair. I actually have troubles drawing without any sort of reference at all (though I'm getting better) and I know that most professional artists usually do a lot of research and get a bunch of reference pieces, not to simply copy but just to see how the world works. How limbs join together, how fabric folds and such.
Another suggestion I can make is to first draw the person naked, taking note of the form. That way you can figure out how long the arms need to be, how large the hips of a girl will be (this is I find the primary thing that can identify a girl, not breasts in fact. Not knowing this is why a lot of guys tend to overemphasize the breasts in their drawings of women.) And then that way you can just draw the clothes on, and you'll be able to better understand what parts of the body are keeping the clothes up and where the fabric should be hanging. Your drawings have inspired me to try drawing for Topaz again! I realize it's something I can do during those one hours breaks I have between classes and I just found out that the library has a scanner. So let's keep trying together :3
Or actually in retrospect, maybe I can try to digitally colour in your drawings. I'm liking the idea more and more...