It really is all about practice as I am finding out myself - I did a commercial premises with my mate and he had to go over some of my handywork because, at the time I was constantly resorting to dabbing the window with my scrim to clear any water on the glass - in effect I was making the whole thing worse - the real answer is to get your appicator to work thoroughly over all of the glass and then work on your squeegee technique until you really just need the tiniest margin of detailing to do round the edge.
Everybody has their own preferences re squeegee size - After reading some of the posts I decided to try a 10 inch over the normal 12 that I was using and it has made the whole thing better again.
Good luck