Whether walking or running, a good technique will help you with performance and also in preventing injuries. One of the most typical mistakes I see in runners and walkers is incorrect arm swing. You want to always keep a 90 degree angle in your elbows and allow your arms to swing from the shoulders. Never should the elbows open up and collapse in while you're swinging your arms back and forth. This will result in just a waste of energy. Instead, keep that 90 degree angle and let your shoulders swing back and forth like a pendulum. When running or walking long distances, this more efficient technique will help you go longer distances before being fatigued.
It's hard to learn how to keep yourself loose when you run. But, it's important to keep trying until you learn how it feels. Your shoulders should be nice and loose. Everything from your shoulders and down to your arms and even to your fingers - loose. It sounds like you could be shrugging, tightening your shoulders while you're trying to work on your posture. Check to see if you're too tight next time you go out. Keep me posted!
Sometimes when I run, I get a cramp in my shoulder. I read your tip, and I feel like I do that. I have focused on trying o improve my running posture, but somehow it seems that something is going wrong. Any ideas?