[QUOTE=basics]These are problems that occur sometimes - the change in playing standing up compared to sitting down and the change from playing through an amp and not playing through an amp. To get around the standing up sitting down issue I always play standing up. You gotta know that if you play sitting down for any length of time when you try to play standing up and with a strap it's going to be different.
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I keep my guitar strap pretty tight, so the guitar is at the same position whether I'm sitting or standing.

I've been having shoulder pain as well. I got one of those Planet Wave straps that's extra wide, so it distributes the guitar weight a bit. Sometimes helps. I'm thinking about getting one of those dual straps that goes over your should and across your lower back. I've heard it provides more stability, but don't know if it helps this problem.

My chiropractor used an ultrasound gadget on my shoulder a few weeks ago. Seemed to relieve the pain for a bit.