How to Stop Shelves From Sagging in the Middle

Empty the shelf of all contents, and use a tape measure to locate the center of the sagging shelf. Mark that spot on the wall (or the back of the freestanding bookcase) underneath the shelf.

Place a metal shelving bracket at this center point, and line it up with the other brackets on both ends of the shelf. Mark where the screws will need to go, and then remove the bracket and the shelf.

Screw drywall anchors into the spots you marked, making sure they are flush with the wall when you are done.

Put the bracket in place, and then put the screws into the drywall anchors. When they are completely screwed in, you are ready to replace the shelf.