Watch the Incorporating Open Strings online guitar lesson by Andy Aledort from Slide Guitar Power
The greatest advantage of open tunings when playing slide is that all of the open strings become available for riffs and soloing. Many of the essential slide riffs and signature slide parts from well-known blues and rock songs are executed with abundant use of open strings. A very simple and easy to remember approach for memorizing an open-string based scale pattern in open E is to fret notes at the third and fifth frets of every string along with the open strings - this technique was demonstrated to me years ago in an interview lesson with legendary blues-rock master Johnny Winter. Practice this first by fretting conventionally and then play the same patterns using the slide. Once familiar with the pattern, play the accompanying examples and experiment with creating your own open-string-driven licks. Another essential focus of study is to learn and play the E minor pentatonic scale in this open position.