Work on your weaknesses

This is something that took me a long time to fully appreciate, but it's really helpful to spend more practice time on your weaknesses than the things that are already easy. I wanted to improve the speed of my scales, so I just practiced my strongest right hand fingering (i & m alternation) figuring that I would just use that fingering anyway, so why waste time practicing the others? Eventually I started devoting more time the weaker combinations, particular m & a, and as a result the independence between my fingers and the overall relaxation of the hand improved tremendously. My whole technique improved much more rapidly than before, including the speed and (more importantly) the ease of my scales, and it gave a big boost to my confidence with the instrument as well.

A technique to develop relaxation

While it's true that there is a component of strength necessary when playing guitar, excessive tension and overuse of strength are common problems amongst guitarists. A practice technique I have been using to address this is to play a particularly tiring piece without actually pushing the strings down. My right hand operates as normal, the left hand fingers touch the strings at exactly the right point of the string, with right spot on the finger, in order to play each note cleanly, but it should remain so relaxed that it doesn't push the strings onto the frets.

When trying this, go very slowly and make sure you get every motion right. In your mind's ear, listen closely to the notes that would be played if you did touch the strings to the frets. I often find that by the end of the piece, I have begun to actually play the notes with my left hand so relaxed that it perceives almost no effort. When this happens, I try to internalize that feeling in my hand and maintain it as I continue to play.