Watch the Big Slide online guitar lesson by Jeff McErlain from Essentials: Solo Electric Blues
For Big Slide I have tuned the guitar to an open E chord. The notes from low to high are E B E G# B E, the same notes as an E chord. Each tuning has its own sound and E is nice and bright. It has the great benefit of having the root on the high string which we will take full advantage of in true Ry Cooder fashion. Open tunings also work very well with slide guitar because if you think of the slide barring across the 5th fret for example while tuned to open E, it would give us an A chord. If we barre across the 7th fret with the slide, it gives us a B major chord etc. Open tunings and slide become easier to understand when we see how they are similar to moving barre chords. Of course that is a very simple view of it all but it is a start as many people get scared by the idea of re-tuning the guitar. I really enjoy it because of the sound it gives us, we could not play a piece like this in standard tuning and have all those great open string sounds and slide chords without it.