I have tendanitis in both of my shoulders, so I tend to have shoulder pain quite often when playing for an extended amount of time. I like using wide, thick, Levy straps. Like the good Dr. said, wider straps destribute the weight more evenly across your shoulder making things more comfortable. Another suggestion would be to stop being lazy and get used to playing standing up. I was the same way though. After playing the acoustic for 5 years and then moving to the electric, it took me a long time to get used to playing standing up, but now I play standing up more than sitting down.