I learned this one years ago and it’s wonderful.
When you are washing windows, it’s sometimes hard to tell which side a streak is on. Well, just wipe one side horizontally, and the opposite side vertically. You will be able to tell exactly which side contains your streak!
But don’t look at my windows. I haven’t cleaned them in a very, very, very long time. They don’t have any streaks, just water spots and grease marks (from foreheads on the glass!).