You will see fighters shaking their hands trying to get their shoulders relaxed sometimes. What works even better is to step back and lift your hand up higher and back towards your ear while trying to relax your shoulder. Also stepping back and rolling your shoulders also works for some.

You hit on a key point though in your last statement and relaxing physically even though you are getting tired. I stress this a lot with fighters and relaxing physically does not mean relaxing mentally. As a fighter gets tired he should try to do things as well as or better than when he was fresh and that takes focus and concentration and yet staying relaxed.

One of the things that can help you with this is in your heavy bag workouts focus on doing this even though you are getting tired towards the end of rounds and the end of your workouts on the heavy bag. You might even take a step back and lift that hand up and back towards your ear; just make sure there is a break in the action when doing this though.
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People who are hunched over have aligned their limbs in such a fashion that it forces them to maintain tension in the shoulders. Simply change the way your hands and elbows are located in a defense.

This is often seen when people are worried about their reflexes in block, so they tend to hold their hands up and maintain power to the shoulder muscles, because they’re afraid of any delays. I suggest using the face and torso guard that doesn’t rely on hand reflexes instead.
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