Particle board shelving works because it is lightweight and economical. It does have a few drawbacks though. The most prevalent of those is its tendency to sag or bow over time and with use. Often homeowners simply place too much on the particle board shelves and they begin to bow or sag within a few months. Particle board is made of wood chips, saw dust and shavings combined under great pressure with adhesive. They are very sensitive to moisture and will swell when wet.
Remove everything from the shelves. If this is a bookshelf, then lay it on its side on the floor. This will make it easier to work on the shelves. If you have a wood shop or garage area where you can work, that would be best.
Cut a 2-by-4 piece of wood the length of the shelf. Next clamp it to the top of the sagging shelf in the middle and at the ends. That will draw the sag straight. Pad the clamp ends in contact with the shelf with pieces of furring strip.
Glue and screw a 1-by-2 strip of wood edgewise onto the lower back side of the shelf with the narrow side fastened to the bottom of the shelf and the wide side fastened to the back board of the shelving, cupboard, etc. The strength of the furring strip now supports the shelf. If needed, you can also fasten a 1-by-1 to the bottom of the leading edge of the shelf, and paint or stain to match the book shelf. This will help keep the front edge of the shelf straight.
Place 2-by-4 pieces of wood between the sagging shelf and the top of the shelf right underneath it so that they work to hold the shelf in place while the glue dries.
Attach the support brackets. If using metal support brackets, then simply screw them on under the sagging shelf in optimum locations where they will do the most good at supporting the shelf.
If glue was used with the support strips, allow it to dry for 6 hours before using the shelves.
Once your sagging shelves have been repaired, do not overload them. Choose light weight items to place on these shelves.