This most recent piece was the first that I had the book for. Before that I had no "official" anatomy reference. And of course, take note that this particular book is for "lifelike" figures, whereas the characters of One Piece that I've worked on come in all sorts of crazy shapes and sizes. But that gives me an idea for a whole new line of pieces: maybe I could make "realistic" versions of OP characters. Like, Nami and Robin without their usual logic defying figures, or guys like Arlong, Magellan, or Moria as normally proportioned people. Might be fun!
As for the structure, everything at this point has a wire armature, but Moria was the first to receive a foil core. Otherwise, I don't treat space filler clay any differently (except that I don't care what color it is).
Clothes are tricky, in that they present different challenges than skin. Folds definitely add to the realism, like you mentioned, but I usually can't achieve more than the basic body contour folds. One problem that I still have is that the clothing is never thin enough (Akainu's coat was a real pain- it was too heavy and started tearing under its own weight at one point); the thickness tends to hide the body's true form more than I would like. Overcoming that would require a pasta machine, which I might invest in soon :D