Theres a method that i use that works great, i call it w's and z's lol firstly clean all the frames as they are the dirtiest usually then clean each window from top down in a w motion and once thats done use a z motion to rinse the dirt off, i have found that if you are cleaning dirty windows that havent been cleaned in a while they tend to run from the rubber seals so just make sure and give it an extra rinse, its really easy and thats why everyone thats turning to window cleaning now buy a WFP system.
On to the other part, if you want to take the window cleaning further then id suggest buying the van system you are on about, a bit more money but you will clean a few more windows in a day when you get built up.