One of the most popular and highly rated books on fashion illustration is the Fashion Sketchbook by Bina Abling. This is a HUGE notebook with almost 500 pages and lots of exercises and examples to use.
One of the lessons that I found most helpful was getting the body proportions right. I have shown an example here, both from the book and my sketch that I did according to Abling's lesson.
One guideline that seems to be fairly universal is that the figure should be about 10 heads tall. I have a tendancy for some reason to make the heads too big, so this rule helps to prevent that from happening.
Abling breaks it down further, suggesting that the shoulders be at 1.5 heads, waist be at about 3.5 heads and the knees at 6.5 heads.
This is an outstanding resource, and if you get just one book about fashion illustration, this is a good one. The only criticism I have is that I like fashion illustrations that are highly stylized, and this book doesn't really teach how to do that. But, for most people, getting the basics first is important, and only then does it make sense to work on the style and personality of the drawing.
Thanks for visiting my blog! If you are looking for a current example of my custom fashion illustrations, visit this link: http://lizblair.blogspot.com/2009/10/alabama-chanin-design-and-my-drawing-of.html