do you practice daily? because i do. i draw everyday, and i have been since i could hold a pencil properly.
my advice? get a light table, print this out, and trace it. trace it again. then put the light table away, and copy it free hand. and again. and again. then put my drawing away completely and mix my style into yours to create something new.
also, invest in an anatomy book, or spend sometime looking up nude pictures and simply observing how a body works. some "How to Draw [blank] Manga Style" would help too (if you're into manga, that is).