My father was an old school cabinet maker. He learned his trade during the depression, a time when everyone learned to be frugal. One of the first woodworking tips I remember him teaching me was the proper way to fold sandpaper. Using this method the sandpaper will last much longer. Sandpaper is expensive, even today.
Step 1: Here's the Issue
If you simply fold sandpaper twice, the abrasive surfaces face each other. While you are happily sanding away with the first side, the two abrasive surfaces are ruining each other.
Step 2: Start With Some Creases
Fold the sandpaper in half, making a sharp crease across the middle. Open the fold and rotate the paper 90 degrees. Fold once again and make a crease 90 degrees from the previous crease.
Step 3: Tear Along a Crease
Using the edge of the table saw, or a sharp corner on the workbench, tear along one of the creases. Any crease will do, I always pick a short one, I'm kinda lazy.
Step 4: Fold Number One
Fold one of the panels over, as in the photo.
Step 5: Fold Number Two
Fold again, as in the photo.
Step 6: Fold Number Three
When you bring the last flap down, notice that there are no abrasive surfaces facing each other. Neat.
Step 7: Sand Away!
Now you can sand to your heart's delight. Unfold and refold as necessary. You can even wrap the sandpaper sandwich around a block of wood if you wish. Notice how much longer the sandpaper lasts, simple, but amazing.