Oxford pillion grip belt or "love handles" are great. Your pillion only needs to reach to your sides rather than reach completely around your waist. My wife won't ride without them.
Also, try a triboseat grip of you don't already have one. They stop the pillion sliding around so much meaning that they expend less energy on trying to correct their posture every 2 minutes.
http://www.halfords.com/motoring-tra...n-grab-handles
http://www.triboseat.com/