To keep your windows looking great, always start the process with a clean applicator and clean window scrubber. If you start with something dirty, your window cleaning will be that much harder. Also, consider changing out that window brush any time that it begins to get too dirty.
If you plan on doing some window cleaning today, make sure it is dull, not so shiny day. Windows are much more likely to streak and look bad if you clean them in direct sunlight. That's because the sun will heat the glass so much so that it will cause the water in the chemical to evaporate too quickly for you to clean it off.