Arrange a pre-set date/day of the month for cleaning the high level windows that every one will remember to close their windows. Also mention to them that if it's too windy for safety resons you will return at your earliest safe opportunity to start or complete the job and stick to it as customers expect a regular routine from you also.
I always start the highest windows first and work down the levels before moving around the next side of the block of flats unless it's a small block of flats in which case I just start at the top and go right around the building, working my way down. It all depends on the buiding and what obstacles and other challenges prevent you from choosing your preferred method.