Careful to never bare a belly or a fake tan, Amy Ray and Emily Saliers of the Indigo Girls are far from a music-industry product. Since the late 1980s, the folk-rock duo has played thousands of shows, catering to a largely female fan following with smooth harmonies and by-the-campfire type serenades. Constant touring and an intimate, hard-to-beat artist-audience bond have kept the Girls in business without the help of a marketing tycoon.