Kitchen Cabinet Shelves Keep Falling. You don't need to use screws at all four corners. Break the toothpicks into short bits about the depth of drywall, put a light coat of elmers glue around the pick bits, and stuff them in the hole till no more fit.
Measure the area where any additional support is needed along the back of the shelf, especially if lack of support was the cause of sagging or falling shelves. You stick them in the cabinet and the pretty much stay there unless you somehow damage them. Failure to address these problems can lead to the cabinet becoming completely detached and crashing into the floor.