Everyone needs to hand sand something at some point. A full sheet of sandpaper folder in half and half again fits your hand and feels about right. But using it like that and half of the sheet will get worn out sanding against itself and not your workpiece. So Columbus showed me this little trick.
Fold the sheet in quarters, then cut it halfway down one of the seams. He used an old hacksaw blade to cut it.
For the first quarter to use. Fold the far left corner down.
Fold that stack over to the right.
Then fold the last single layer over the other three.
Ready to go and no rough sides touching each other. Use the two outside quarters up, then unfold and refold starting with the upper right first. And you have two fresh quarter sheets to use.