if your having shoulder problems then you ve gotta use xtreme brushes on the end of your xtreme poles mike.they make a big difference.
a medium mixed brush would suffice and rinse on the glass for upper windows.if you get the angle right on your resi neck you can clean frames,sills and glass effectively on maintenance cleans.having a gardiners scraper on your pole helps with the more stubborn marks too.
i d also refuse any f/s/g and conny roof jobs if i were you as these put a lot more strain on shoulders than regular windows.
cod liver oil and glucosamine every day(i swear by it)
learning a few light yoga exercises to be done every day would be beneficial.even 10 mins in the morning and evening would do you good.
ive never had any shoulder problems for years now apart from the odd ache or little niggle(usually from squeezing in add on jobs on an already very busy window cleaning week).
i already have big strong arms and shoulders from playing drums and gym work over the years.im also 6 ft 2in and 17 stone and have been working outdoors all my life in manual jobs(farms/landscape gardening etc)
i also love swimming but dont go as often as i used to.
when im relaxing at home i slouch on the sofa no problem.i certainly dont think about how im sat or whether my shoulders are back or forward!

im 44 now but i feel much better than i did the whole of my 20s