Hello! I took a brief look and I have a suggestion on some things.
First - The mermaid from March 30th.
Just a couple tips! (But first a compliment, I really like her tail!) When you're coming up from her waist to her ribs and over her back, most women don't tend to have such a harsh separation there between their waist and ribs (unless they are very thin). Her ribs are also a bit thick at the bottom. If you tapered them off, it would fix both issues.
I recommend also using a bit more under-drawing. Don't be afraid to sketch out using shapes to establish the proper body shape. I usually start with a line for the spine, then I add two triangles tip to tip (like an hourglass) to help define where the shoulders and hips are, which are both key (even with a mermaid!) to placing the body well. Here's an example (not my work) of under drawing: http://i47.photobucket.com/albums/f167/tloz123/shapes.png?t=1281588743
There are several ways to do it, so feel free to find a book or look over more details from other people on other methods.
Another small detail. Breasts tend to be less rounded and more teardrop shaped, or pointed where the nipples fall.
I really like your toned paper anatomy sketches. They show a great understanding of anatomy and movement. Your mermaids are wonderful pinups. I admire the way you can capture poses with little to now scratchy lines I often find myself with rough workings of a sketch then I erase everything and use the halo lines to create my longer poses to get the best anatomically correct pose. That method also takes pressure off of needing to nail it the first time around.