Concentrate on frames especially top, over-do them as much as possible. then do glass, rinse and rinse again. Repeat for all windows. Then (if neccassary) go back and do glass only without touching top of frames.
Some people say that it takes a few cleans to get it spot free, Thats rubbish!! It just needs to be done thoroughly. Remember, customers are most likely to inspect the first clean you do so it HAS TO BE RIGHT. once they are happy with the first clean they are more trusting that you have done a good job and wont bother inspecting in future cleans. You do a bad job at the start, they will inspect everytime (if they bother to keep you on)!
Just dont rush the job and take the time required. If you do this you will fly through the 2nd, 3rd, 4th clean