Don't forget learning to transpose between keys. You might have to play the same songs with different singers in different keys. So learn it in different keys. You'll pretty soon learn how keys relate to each other. Use this along with the methods described as there are transpositions that are awkward ( you don't want to be playing in Bb and Eb just by changing to those chords....that's A and D with capo at fret 1) Of course this is a bigger deal for lead guitarists as you have to learn the chords and lead lines.
There's also considering tuning guitar down if it suits your regular singer. Tuning down a half a step is a common practice for live playing as it takes a bit of the strain off the singer.