Watch the Extensions online guitar lesson by Sonny Landreth from Slide Supernatural
Most of the time when I’m playing slide I use the slide as the anchor, so to speak, and I’m using my fingers behind or playing phrases of melodic lines, and it’s still based on keeping the slide and everything in place. But another way to do it is to use what I call extensions, where the index finger becomes the anchor and then the slide is used to play the melodic line or a chord, or to imply a chord in using chord melody. One example would be the intro to the song "Brave New Girl" and what I’m going to do is hold the C sharp down with the first finger of my left hand and I’m going to bend in and out with the slide to make this melodic figure on top. It’s a little tricky at first because you don’t have the gravity working for you having the slide in place so you’ve got to lift it up and still keep control with the third finger and it does help to develop an independence with the fingers so you can plant this here and move the slide on top.