GLAMNICISM!!!! Repost (properly) previously posted 12/6/15 and failed to include video.
I have been sketching alot for an upcoming project. The below video is the best method to understand the body in static fashion poses that I have found. This designer/illustrator has many videos where she uses this same method for building a basic croquis portfolio. I suggest finding photos of models in resort wear to be able to clearly see the body curves knee and elbow placement etc. (a suggestion I learned from Ronaldo Barnette a successful artist/illustrator/designer living, working, and teaching in New York City).
There are many videos on YOUTUBE by many illustrators noteably, Bina Abling, Kathryn Hagen, FSketcher, and the list goes on. The very most important element in becoming a GOOD freehand fashion illustrator is PRACTICE, PRACTICE, PRACTICE, PRACTICE!!!!