Exploring my own tension habits on the guitar has led me to spend copious hours over the years re-working my technique and movements, and studying with an Alexander Technique teacher (AT is all about breaking habits of tension). I also notice excess tension in many of my students, and work with them to change the habits that lead to it.
The question comes up quite frequently, and so I decided to weigh in publicly on the issue.
Here is an off-the-cuff video with some of my thoughts around tension, and what to do about it. While this video deals with tension in general, I will add that developing the right fundamental techniques and movements is also extremely helpful (or at least not harmful!).