You need to sit your saddle further back to ensure it is off the left shoulder which is probably bigger. As the left shoulder comes back it will be pushing the saddle over to the right so no girth straps will make a difference. You are also probably right handed so are stronger through the right and therefore you are more likely to put your weight across to the right compounding the shoulder problem.
So sit your saddle slightly further back, i presume you have 3 standard girth straps plus a point strap ( ie an extra front strap), use the 1st and 4th strap on both sides. get your self a stubben cord girth to spread the weight across the widest area and minimise movement.
Thanks I will try that. I have just bought a stubben chord girth but it didn't help when I girthed up using the middle two straps. Today's quiet walk out resulted in me hanging to the right again.
My saddler when my saddle slipped to the right a bit (when he was coming back into work after box rest) said to use the 1st and 4th on the left and 1st and 3rd on the right. He also said avoid any elasticated girth. A limpet pad could also be an option