With prolonged usage, zippers can become loose and begin to "unzip" themselves. There are several DIY ways to fix a zipper that won't stay zipped up, such as using pliers, nail polish, hairspray, a hair tie or a key ring.
Fix a Zipper Using Pliers
One of the most common causes of a zipper unzipping itself is the zipper slide, the portion of the zipper that moves up and down. Over time, zipper slides begin to stretch and warp. You can use a pair of needle nose pliers to pinch a zipper slide back into its original shape. Begin by pinching one side of the zipper slide gently with the needle nose pliers. Test the zipper to see if it still separates or falls. If necessary, pinch the other side of the zipper slide with the pliers. This returns the zipper slide to its original shape, which helps it stay up.
Fix a Zipper Using Nail Polish
If you have a small zipper that won't stay closed, such as on a pair of jeans, you may be able to help hold it in place using clear nail polish. While the zipper is open, paint both sides of the zipper teeth with nail polish and allow to dry completely. The added bulk keeps the zipper teeth locked together once the pants are zipped up. You may need to use several coats of clear nail polish, depending on how loose the zipper teeth are.
Fix a Zipper Using Hairspray
A temporary, short-term solution for a zipper that won't stay done up is to spray the closed zipper with a strong hold hairspray. The hairspray acts as glue, holding the zipper teeth together. This process needs to be repeated each time you open the zipper, or wash the item. This fix works best on short zippers.
Fix a Zipper Using a Hair Tie or Key Ring
A last resort option for fixing a falling zipper on a pair of pants is to use a hair tie or key ring to hold the zipper slide in place. Thread a small hair elastic or key ring through the zipper slide. Pull the zipper up, and place the hair elastic or key ring around the top button of the pants. This keeps the zipper from falling down, and should be almost unnoticeable when the pants are buttoned up.