1) start with a full sheet of sandpaper -- cut into 4 equal pieces
2) fold and score each quarter sheet into quarters - cut one score just to the middle
3) fold one flap up, then over, and down -- now you have 4 clean sides of sandpaper -
you can fold and re-fold to a clean side as needed
Remember - the only area of your piece that is being sanded is the space just under your fingertips ...
Sand in the direction of the wood grain - but align your fingers perpendictular - so that your fingertips are moving sideways with the grain and not up and down with the grain -- the reason is that each fingertip has a different strength and applies different pressure. If you go up and down you will essentially create valleys.
Not only will these tips give you a better finished product they will also save you money