I have a black mens version, works well, good price.
Sew or glue a sticky rubber to the shoulder strap of your pack. There is a rubber they sometimes put on shelves in kitchens that works. other options might be to smear some RTV (silicone adhesive) on the straps.
Then, it wont matter what you are wearing.
Have you thought about going in a sewing store and buying velcro? Attach a piece to your jacket and put the other piece on the strap of the camara, when you walk around it will stay put. When you need it, it pulls off very easily.
My now 27-year-old Domke photographer jacket has a thin strip of rough material on the top of each shoulder -- which keeps bag straps from slipping off of the shoulder. An earlier post mentioned Velcro all you would need is one part of the pair of Velcro strips -- perhaps the loop part of the pair -- just the something that has enough roughness to keep the strap from slipping. Again, a very thin strip is all that is needed.
The old military jackets had big buttons sewn on the shoulders. They also came with flaps that could hold gloves and hats and things, but the buttons were there mainly, I think, to hold the straps of the various bags that would be carried.
I believe deltoid excercise would help.
I am thinking about the rubber and button solutions.