One of the central techniques to get this riff right is to keep the right-hand moving using down and up strokes. The motion locks in with the proper picking pattern we need for the riff should we decide to double pick it. If you want to give it a little bit more of a Led Zeppelin style vibe, don't double pick it but keep the right-hand moving at half the speed.
The riff is based upon an E minor blues scale on the I chord. For the IV, take the same exact riffs and move it towards the floor one set of strings. For the V chord, the riff gets a little more chromatic by moving right up B D, D#, E, F, and F#, then moving down a whole step and playing the same thing. Although the riff appears to be fairly simple, to get this to sit right in the groove and be really aggressive takes some practice. Sounds great with the band and make sure that drummer keeps the cymbal wash going!