1 cup lukewarm (NOT HOT!) water
1 tablespoon cornstarch (cornflour/Maicena)
1 clean cloth such as a piece of old towel or rag
1 microfibre cloth
1. Shake up water and cornstarch to mix well in spray bottle. DON'T use hot water, or you'll get a gluey mess. Just lukewarm.
2. Spray this mixture on dirty windows and scrub with the cleaning cloth. The cornstarch breaks down the grease and the water moves the dirt.
3. Buff dry with a microfibre cloth. There should be no streaks to deal with!
If you're doing both sides of the windows, do one side vertically and the other horizontally; that way you can see where it is if you missed a patch.
Source of recipe: Someone gave me this cleaning tip some time ago and it works.