There is nothing more beautiful than material that is painted well. A painting can be breathtaking when an artist can translate the textural feeling of material in a way that not only pleases your sense of need to touch, but also stimulates that part of your sight that is so fond of color and effect. They might paint what they see like a light reflecting in jewel patterns on the sheen of the material, a white that reflects a rainbow of softly woven reflections of all the colors in the prism of light, a beauty of a pattern recreated by their paint brush in masterful strokes, or the curvilinear lines that describe the sensual flow of a enchanting form under a piece of material. Whatever the effect or subject, painting material is a magic form of adding intimacy to a subject. Material is just as much part of a person's character as their hair, their features, and their hands, because it is what holds to the shape or lets loose of their form in response to their personality. So, what and how you paint the clothing and materials around your subject(s) is as important as all the other features. Some painters really miss this, resulting in paintings that are stiff and unfeeling.
As a special note, you cannot paint clothing until you learn to paint the human figure. So put your time in studying the human form, skeleton, muscle form, and anatomy in any and all ways you can, prior to painting clothing.
When painting the folds in cloth or clothing, be sure to make your brush strokes move with the direction of the flow of the material. For example: if there is a hump or a fold in the material, the brush stroke should come from the top of the material. Remember that the material does not just end at the wrinkle, it continues on under. Use your mind and your hand to create the motion in your material by seeing around the edges, proving the shape of the rolling material in the way that the value is applied.
Material is a supple object. It will therefore take on any shape that it lays against. Thus, you must understand what it is laying against and show the form or shape of the object with purpose. If you can do this, you will carry out a moving piece of art. You may get tired of hearing me say this, but you must think like a sculptor. Think three-dimensionally. When at all possible, have a model or some form of a modeled human anatomy around to remind you of the shapes of muscle and bone structure.
The material has a character of it's own that you also need to take into consideration.