To get windows perfectly clean you must remove all of the dirt from the glass. Professional window cleaners get glass clean by doing just that. They use a sponge or wand to apply a cleaning solution. As they wipe the glass, an enormous amount of the dirt is actually transferred to the sponge. The solution is usually just a mixture of water and a mild soap de-greasing agent.
Professionals use squeegees to remove the solution that is left on the glass.
The soap in the solution lubricates the glass surface allowing the squeegee to glide across the glass. To achieve professional results, you need to use the right squeegee. A professional squeegee is often constructed with a brass channel that holds a rubber blade. Brass is used to hold the rubber blade because it is a soft metal that will not scratch glass. Some hardware stores may carry professional squeegees.