Sometimes you can get away with slicing off a thin piece of sandpaper and folding it up to fit into narrow spaces when detail sanding, but what you really need is abrasive cord. It’s already round — which means no folding — and it’s much stronger than thin strips of sandpaper. It’s cheap, too.
You can find abrasive cord at most any woodworking supply outlet. Rockler, for example, carries it in .055″ and .070″ diameters as well as a 3/32″ flat version which they call “abrasive tape.” They also offer a variety of grits.
And since a 12′ spool runs just $6 at Rocker, there’s really no excuse for not keeping some of this handy around the wood shop. I also found a 50′ spool for $12 at Micro-Mark, which means those of you doing a lot of detail sanding can easily find for cheap purchase it in bulk. Check the street pricing and Amazon links below for numerous other providers.