Hi all, thought I'd toss in my two cents on harmonica accompaniment for guitar.
Here's a cool tip for finding the right key for playing cross harp: start with whatever key your harmonica is in, and then count up four whole notes.
For example if you have a harp in C, counting up four notes gives you G -- C, D, E, F, G using G as your root chord, try playing a 12-bar blues progression using the I, IV, V format. In this case G, C, and D
You can do it in reverse, too. If the song is in E, for example, count back four whole steps to A. So the chords are E, A, and B (I, IV, V) and you should select an A harp
Without getting into too much music theory, the root chord (or tonic) is position I, position IV is the subdominant chord (the key your harp should be in), and position V is the dominant chord.