Here's what I say:
Learn to play dominant 7 chords. That's what blues ( and hence, rock) are all about.
Take your average IV, V, I progression, i.e. A, B, E. Now play those as all dom7 chords, i.e. A7, B7, E7. Then you can play a pentatonic E minor over them, and you have something that's going to sound far more interesting that just power chords.
You may notice that the dom7 chords have a major 3, and the minor pentatonic has a minor 3, but that what makes the blues work! Crossing those 3rds is what makes it really blues!
Once you learn to this, you will find yourself playing stuff that sounds much more harmonically complex, and you will hear that it sounds much more like SRV or Hendrix and much less like (insert '80's band here).
If all this is totally foreign to you, then get a theory book and start reading. Some times the best hour you can spend practicing is training your head, not your fingers.
Best of luck,
Aaron
http://www.voodoovibe.com