try a bit of stretching/yoga in the morning before you start and again in the evening.even 10 mins will make a difference.
dont grip the pole too tightly
use a rocking motion when cleaning upper windows.
take cod liver oil with glucosamine every day
get a massage a few times a month
dont work more than 6 or 7 hours "on the glass" a day 4 or 5 days a week
have a few holidays abroad a year in 4 or 5 star hotels with jacuzzi/health spa etc and RELAX,swim and light exercise in the gym.eat well etc.fully recharge your batteries!