Well, two things are good.
1) practice using your pinky on power chords....
so rather than playing
E- - -
B- - -
G- - -
D- 5 - (4)
A- 5 - (3)
E- 3 - (1)
(brackets = fingers used)
try
E- - -
B- - -
G- - -
D- 5 - (4)
A- 5 - (4)
E- 3 - (1)
you don't need to play that second G (5th fret, D string) when playing a power chord, as it just completes the octave.. BUT by using your pinky to play it you will help improve the strength in your pinky...
and then another exercise to improve strength in all your fingers (this might not make sense, but it's a good practice tool)
this is also used to increase speed and accuracy.. but most importantly it builds up wrist strength, endurance, AND helps to strengthen each finger (especially the most underused pinky finger)...
it is difficult, and it WILL hurt the first several times you attempt it, but it works really, really, really well.
Anyway..
start on the low E string.. you will be progressing from the low E to the high E (so playing each string)
And go fret by fret
So, if you're looking at the neck upside down, you're going.
E - 1 2 3 4 -
A - 1 2 3 4 -
D - 1 2 3 4 -
G - 1 2 3 4 -
B - 1 2 3 4 -
E - 1 2 3 4-
Now, each fret is also the finger you use..
so for the first fret on each string you use your index finger,
second fret on each string your middle finger
third fret your ring finger
fourth fret your pinky.
So far, it seems easy. (heh trying to make this like a lesson here

)
BUT, there are two things you msut attempt to do.
1 - press down on each fret as hard as you possibly can.. literally until it almost hurts your hand from applying so much pressure, and makes your fingers sting a bit (from the strings digging so hard into your fingers).
so apply maximum pressure on each note you play....
2 - ONLY lift off your fingers when they are moving to play the next string..
so when your pinky strikes down, you are holding all four frets down with all four fingers..
and then, when you are ascending (or descneding) three fingers stay in place while you move hte next finger in the progression.
so if you're ascending.
E - 1 - 2 - 3 - 4 -
A - - - - - - - - - -
and then
E - - - 2 - 3 - 4 -
A - 1 - - - - - -
and then
E - - - - - 3 - 4 -
A - 1 - 2 - - - - -
(and so on)
do this ascneding, from low E to high E
and then try the same thing backwards (descending)...
it's harder than it sounds.....
(if this doesn't make sense, just reply an di'll try to make it clearer).
"There must come a day... when the unfinished work of our heroes, must truly be our own."
-Ben Harper