What I do on every new clean. is first, really drub the frames down first, you then use the brush head to remove any drips and excess water from the frames, this stops them dropping on to the bottom windows sills and splashing the bottom windows after you have left.
I then clean the glass in gentle up and down movement, any bird crap or dirt which is still left, I scrub a bit harder or use a scraper which attaches to the pole.
I then rinse off the glass.
Do the tops, front and back first, making sure that you wipe the tops windows sill of any excess water.
I then repeat with the bottom windows, if any window has spotted I then re-wet the glass blade that window dry.
On you next clean you won't have to do this again.
You can get spots for various reasons, the main cause is that the wind blows dirt particles onto the glass when its still wet, this drys and spots show up.
Always get you customers to let you know that if this happens , you will come straight out and re-clean the windows.