I got my costuming start in the Society for Creative Anachronism. After a few years of knocking around in T-Tunics, I finally hit on cotehardies as a comfortable and attractive medieval style, and set about designing my own pattern, loosely based on the 14th century gowns discovered in Herjolfsnes, Greenland. Ironically, it wasn't until I'd stopped being involved with the SCA that I went on to do a full court Elizabethan gown.
I don't do a lot of specifically accurate historical costumes, but I have found that doing the research has given me a good foundation in the basics of garment construction. Even if I don't deliberately set out to make a historical outfit, it's helpful to look at the shape of something, and know it's basically a cotehardie, or that it must be held up by a bumroll or what-have-you.