Breathing now and then: shrug your shoulders to relax your muscles and taking it slowly to begin with. You said you were going slowly but I mean, get up, go do something else for a few minutes and come back to it. It can take a while to learn to relax from the top of your head all the way down to your toes.
Is your sewing machine bed a little lower than the bend of your elbow? That can make quite a difference.
I recently took a class from Barbara Shapel. I also purchased her video for my home reference. She taught us to hold the quilt more like we were carrying a bowl and move the quilt around. It takes the strain off the shoulders, especially.
Here is a quick video clip. At the 4:40 minute point, you can see how she holds the quilt sandwich. In the video she actually explains it. This has helped me greatly to relax more. Hope this helps you.
http://www.youtube.com/watch?v=dCLTPQm2o4U :-D