I get this in my shoulders- if I don;t watch it, they are sometimes up by my ears I am so tense! Being conscious of it helps. Like for me, it used to be that whenever I wrote an emaila t work, I'd check my shoulders, and if they were tense, I'd relax them. This helped them start being relaxed more of the time.
Also, Progressive Muscle Relaxation helps. Tense those muscles consciously for 10 seconds, then release them and relax them for 20 seconds.
Also, deep, abdominal breathing. Imagine relaxing everything as you exhale. Do ths for a few monutes every day.
All of this will help train your muscles to be more relaxed and to let go of the tension you are feeling.