Sandpaper. Who spends loads of money on sanding blocks, or sandpaper that just doesn't seem to last? The cheapest option is to use sheets of sandpaper, it is much more cost efficient than most sanding blocks, but when you fold the paper in in half, it's hard to use, it slips and slides and is just awkward to use.
There are two solutions at hand. The first is a quick fix easy solution. Just fold your piece of paper into thirds. This stops the paper sliding back and forth on itself and makes it easy to get the traction you want on your sanding surface.
Whilst this option is great, the only negative is that as you are folding the textured sides onto each other, while you are sanding, it can erode some of the grit from the internal folded part. If you want to be a little more frugal there is another way.
Fold your sanding sheet in half, making a good line. Then open up and fold in half the other way the same. Then make a tear along one of the folds to the centre.
You are now ready to fold your paper. Starting with the paper grit side down and the torn flap on the top side, fold the flap that has been made from the tear down onto the opposite side.
Now fold across to the left hand side.
Finally fold the last flap down over the other 3 pieces.
By doing it this way, you avoid the friction of grit against grit, thus making your sandpaper last longer.
You can use as it is or fold around a sanding block as you prefer. When the exposed piece becomes worn, simply refold to expose a fresh piece of grit.
I found a great little video on youtube that demonstrates this
https://www.youtube.com/watch?v=fHErFF3QDc4