In order to find your fashion Illustration style you must dive deep into what you like. Practice trying other styles to see what your style is. You will have to train yourself and practice these skill to master fashion illustration.
The best way to improve your skills is to collect swatches, organize and store them in your notebook or folder, analyze them, burn them, test them, draw them. When I say test them I mean see what that fabric can do. Go Play!!! Can this fabric drape well, can it be manipulated easily, does it flow gracefully. Your job as a illustrator is to convey the feeling, movement of the fabrication and design. You must be able to show what the fabric is saying and express it through illustration, go be great! Remember have Fun!!! After all you are a masterpiece, and a work in progress as well. Enjoy your exploration in the beautiful world of Fashion Illustration.