As far as how to do them, you need to deaden (is that a word?) the strings. To deaden a string all you need to do is touch it with your fingers. With this in mind, when you are playing a power chord, DON'T move your fingers, just lift them up slightly so that you are still touching the strings, but not on the frets.
When I play it, my fingers stay in the exact same spot over the frets I need to play. It is just a matter of applying pressure and removing pressure with the fingers on your left hand.
Practice playing a chord (F5 for example) while muting all 4 lower strings. So strum all the strings with your right hand, but the only strings ringing should be E and A. DGBandE should be muted by your pointer finger. It is applying pressure to F (first fret E string), but it's just layed across all the other strings. Not barred, just resting on the strings to deaden them. Then just release pressure from your 2 fingers playing the F5 chord.
So to summarize, just practice changing the amount of pressure you apply to the frets. This way, you don't need to worry about "getting back to the power chord fingering" because you should not be moving your fingers. They will be right where you need to them to be over the frets ready to come back down and let the chord ring.
