One simple feature is my "ComfyCorner". This is a permanently attached armrest which actually wraps around the edge of the guitar. Gone is the painful crease that often develops on the forearm after prolonged playing. The ComfyCorner also slightly elevates the arm off the soundboard and reduces soundboard damping.

Another solution is The Manzer Wedge. Originally developed by Linda Manzer over 20 years ago, this feature involves tapering the body of the guitar from treble side to bass side. The result is a guitar which is much thinner under the right arm, thereby easing arm and shoulder discomfort.