Everyone has owned that one pair of pants that just don't want to stay zipped. They're sleeping on the job and flashing your underpants in the process. Here's how to fix a zipper that won't stay up.
Why Your Zipper Won't Stay Up
The reason your zipper won't stay up is that there isn't a whole lot of pressure at the top of the pants, where the button is helping to keep things closed. However, the middle of the zipper is being stretched by your hips. It has to flex every time you site, stand-up, or stretch. This difference in pressure is a recipe for a lazy zipper.
A popular life hack for a disobedient zipper is to attach a keyring to the zipper pull, then hook that keyring to the button on your jeans. This won't exactly work for slacks or jackets, but it's a good tip to keep in mind if you need a fast, temporary fix for a wayward zipper.
For a stuck zipper, you have to grease the teeth. For a slippery zipper, do just the opposite. Use hairspray (cheap Aquanet will do!) to coat the top teeth of the zipper. Apply only to the top inch or so of teeth. The goal here is to gum up the works so that the teeth have more bulk to them and the zipper has trouble going down.