Blog post alert: Sarah's Questions about Costumes and Writing at Gurney Journey is full of suggestions to aid artists in creating images of clothes, especially for imaginary worlds. Some of the ideas and links can be adopted in making up clothing for characters in fiction.
For written description, actual examples are helpful. They don't even have to be physically present. A dress that Augusta St.-Gaudens described in a letter provided me with Effie's idea to invert pleats in a dress for Jeanette. Images in clothing histories, paintings, fashion magazines, period photographs, sewing patterns, etc., can supply much detailed information and help visualization. Theatrical designs, like the one by Katie Malloy shown here, may have fabric swatches, which even in photographic reproduction can be informative.
Graphic artists need to see the way real cloth drapes, its textures, its appearance in different lights, its complexities of color. The Académie Julian boasted that its collection of costumes for students to study was the biggest in Paris. Gurney's post suggests sources of costumes for present-day illustrators, ranging from thrift shops to local theatrical companies' costumes to museums—and a bonus for writers is that they work for us, too.