I am curious what factors contribute to fabric drape. I know that different fiber types and weave structures make a difference, but what are some general guidelines? For instance, does a plain weave give more or less drape than a straight twill? Is there a way to get good drape from an overshot?
This is a good question and one that you’ll answer more from experience than from asking someone. Three basic factors affect drape: yarn/fiber, weave structure, and warp/weft density. Some fiber types are softer and more pliable than others (compare wool with linen, for example), and highly twisted yarns are stiffer than loosely spun yarns. (Fiber type affects the way a fabric feels beyond just drape; linen has drape, for example, but it has a completely different feel from silk, wool, or cotton.) Weave structures with floats have more drape than plain weave, so in general twills are good choices for garments and scarves and shawls. Probably the greatest influence on drape, though is warp and weft density. The more widely spaced the threads, the more drape the fabric has. It is generally believed that if the warp yarns are slightly more dense than weft yarns, the drape is better in the warp direction.
Determining exactly how to get the drape you want with specific yarns and weave structures can only be done successfully with sampling, however. An easy way to start is to thumb through projects in Handwoven that are woven using the yarns you are considering. A look at the photo can usually give you an idea of the likely fabric drape.