If you're tempted to clean windows (and really, who isn't?), save the task for a cloudy day. Why? A sunny day may cause your window to streak. This is because your cleaning spray will often dry on the window before it can be wiped off.
Remember to clean the windows on both sides, using horizontal strokes on the outside and vertical on the inside. This way, if any streaks do remain, you'll know which side needs to be touched up.
Do you have any other clever tips for cleaning windows?