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.