As far as drawing folds is concerned, I'd suggest getting a sheet and draping it over a chair in various positions to draw for practice, it's a good way to get a feel for how cloth folds and moves. Just keep in mind that cloth tends to wrinkle most around areas where the object it's covering is bent (elbows, knees, underarms, etc) and less on straight areas (upper and lower arms, chest, etc).
This is a pretty good tutorial: http://www.scribd.com/doc/2062815/How-to -Draw-Folds-and-Clothing
I like your stuff, your last piece in particular is looking really good. :]