cjurakpt suggests some good sources. If you wish to try something outside of the trademark licensed schools here are a couple of suggestions.
Learning to relax a certain part of the body might be easier if you tense something else. I suggest upper trap/neck stretches to remind you how much distance there can be between the shoulders and head. Then try to utilize the scapular stabilizer muscles in your back to keep the shoulder blades from floating up when you punch. Some schools advocate punching with a "long neck and short trunk." In other words tightened abs are balanced by tightening the back a bit, giving the sensation of a depressed or compacted ribcage. With the shoulder blades stabilized and "attached" to the mid-back the punch is delivered first with body rotation and trunk power. Then the arm is extended with whatever kind of exertion the style demands. (I know some instructors advocate arm extension first, followed by body rotation--if this is your style you still might try this as an alternative training method.)
Many people have trouble getting in touch with these mid-back muscles and can't feel what they need to do to isolate them. Try depressing your shoulders while remaining relaxed otherwise.
Another technique is to punch slowly while imagining that the shoulders and elbows are very heavy; solid gold, if you like.
Good luck.
"Look, I'm only doing me job. I have to show you how to defend yourself against fresh fruit."
For it breeds great perfection, if the practise be harder then the use. Sir Francis Bacon
the world has a surplus of self centered sh1twh0res, so anyone who extends compassion to a stranger with sincerity is alright in my book. also people who fondle road kill. those guys is ok too. GunnedDownAtrocity