When a metal zipper won't stay up, you risk showing more of yourself than you want to. Flashing a glimpse of undershirt or underpants can be embarrassing. You might think you need to toss the garment, but there is a quick fix to keep zippers in their place. Repairing loose zippers that slide down takes a couple minutes and doesn't require fancy sewing or mending knowledge.
Locate the flat metal piece on the back of the zipper slider, or the part that moves up and down the teeth. This is what you'll be adjusting.
Grasp the top of the zipper slider using needlenose pliers. The pliers should clamp over the front and back of the slider.
Squeeze the top of the slider to push the metal closer together, preventing the slider from creeping down. Use even pressure so that you don't push one side farther than the other.
Grab the bottom of the slider using your pliers. Again squeeze the metal closer together.
If you squeeze the slider together and it still falls down, try it again and squeeze a bit more. If you have squeezed the slider thoroughly but your zipper won't remain closed, you need to replace the slider.