Watch the Duane Allman online guitar lesson by Jeff McErlain from Kings of Tone
Most slide guitarists use open tunings, Duane used the very popular open E. Basically we are tuning the guitar to an E chord. The strings are as follows from low to high: E, B,E,G#, B, E. Take your time with slide and open tunings, they take a long time to master. The technique of playing slide is quite challenging as well especially when it comes to intonation. I highly recommend Geoff Hartwell's TrueFire course Electric Slide.