Like many slide players, Duane primarily used the thumb and first finger of the picking hand. This not only provides more flexibility with expression and dynamics, but also allows for better muting and clarity between strings. As a general rule, the flesh of the first finger works best, rather than treating the thumb and first like a virtual pick. The fretting hand is also important. What many guitarists forget is to mute the strings behind the slide. If you use the slide on the third finger (many favour this or the fourth finger), then place the first and second fingers lightly on the strings at all times. This will avoid any noises creeping through from the strings below the slide.