Sandpaper is usually folded to a convenient size for hand sanding. However, folding and then folding again creates a lump on one edge that sands unevenly.
Fold the paper in half, open it back up, fold it in half the other direction then open it. You now have a piece of sandpaper with fold creases forming a cross in the middle. Tear one fold crease down to the centre like in the photo. Now when you fold the paper, there will be no lump and you can use each of the four surfaces in succession.
Someone at a home show came up and showed me this one. So I pass it on to you from the anonymous donor.