I equate the involuntary tensioning of the shoulder muscle with a flinch. The flinch is a learned behavior really and it takes time and practice to remove it. It really is a mental game you have to play with yourself. A ton of dry fire can both help and hurt you when you are chasing a flinch. Yes, dry fire will allow you to groove the trigger pull and increase the awareness of when the trigger breaks. It will also help you work out the involuntary closing of the eyes prior to releasing the shot. The trouble with this is it, familiarization with trigger break, is that it will lead you to anticipation of the recoil and tensing of the shoulders.
What I have found is that adjustment of the trigger to allow a slight bit of overtravel after the break, I'm not talking about a inch of travel but just a little. You then place all your focus on feeling that bit of travel and pinning the trigger to the rear. That leaves you with something else to concentrate on than just the break of the trigger. By the time you feel that trigger lock back onto the trigger guard, the recoil impulse has come and gone. A box of ammo should do it. If you find yourself back sliding, get off the gun, stand up, take a couple of deep breaths, rebuild your position and with more determination, go back to your focus.
If your gun is hurting you or pushing you around, find someone who knows about gun fit and work with him or her on getting your gun to fit you properly for your style of shooting. You may have to adjust your style a bit also but the gun needs to fit to make things less difficult. I'm fairly long and slender at 5'10", 160 lbs with a 33" arm length. 5 years ago, I weighed 245 lbs and needless to say, my guns from 5 years ago don't fit me very well anymore. Musculature has changed as well as the amount of padding around my upper chest and shoulders. I have had to adjust LOP and comb height to get things back on track.