When you have to sand in a tight corner, it can be tough to reach in there to begin with, let alone to sand with the grain. Pick up some adhesive-backed sandpaper or an adhesive-backed sanding disc and wrap it around a pen, pencil, dowel or putty knife, trimming the edges flush with the tool. That should help you reach where you need to get without damaging surrounding wood. Alternatively, grab an emery board (you know, the kind to file nails with) to reach into crevices. You can find emery boards in varying grits in your local beauty supply store or drug store.