Sometimes when sanding complex shapes, or sanding shapes with sharp edges, your sandpaper will crack and become useless. When this happens you have two options. The first is to switch to a cloth-backed sandpaper instead of, well, a paper-backed sandpaper. The cloth-backed sandpaper is much more flexible and will not crack or rip when used on complex shapes or sharp edges. If that's not an option, the other trick you can try is to apply packing tape (preferably the kind with fibers in it) to the back of the sandpaper. This will help maintain the integrity of the paper backing which helps reduce cracking or ripping.