| I play a five-string, and for power chords I stuff my thumb between the B and E strings to mute them both (if it were a four-string I'd just 'use the E as a thumb rest' to mute it - I usually don't rest my thumb, but keep it trailing along, damping the strings that are above the one I'm playing), and then strum by flicking my middle finger out (so the nail contactls the strings) on the A-D-G while fingering a 1-5-octave chord. I do this pretty forcefully, like flicking a field goal in tabletop football; I used to get a cramp in my hand from doing that for very long, but I can manage it pretty well now - at least for a whole chorus or something, but I don't really use it very often; for a couple choruses on a Meatpuppets song, and I don't remember what else. Once I pick up a Boss bass distortion pedal, then I think I'll use that technique more often; for backing up guitar leads in our three-piece rock band.
I consider your term 'power chord' to mean sounding more like the third-less hard-rock guitar chord (I generally take the 'power'-part to mean no third) - else I'd use thumb, index and middle, and play more like classical guitar, or Nashville chicken-pickin style.
Joe | 
