As a development from my post on PhotoShop, here is what you can do when you add a little drawing into the process.
I started with one of your photos (thanks for posting Georgie)
I then elongated the photo slightly in PhotoShop, printed this out and using a lightbox created a pencil structure for my illustration. As I wanted the illustration to be a potential template for a variety of designs I simplified down to the underlying body shape, I then used fineliner and some acrylic washes to complete the drawing.
I then scanned my illustration into PhotoShop, and using techniques similar to those detailed in my previous post created a garment illustration. With this template I could relatively quickly produce a series of design proposals.
It's not rocket science, but remember the better the drawing you scan the better the final result will be!