Attaching a key ring to a zipper pull and hooking the key ring to the button of the jeans helps to pull the zipper up. Coating the zipper with hairspray can also keep it from falling down.
Apply hairspray to the top inch of the zipper teeth. This gives the zipper teeth bulk so they are less likely to allow sliding. On the other end of the spectrum, when a zipper is stuck, drawing on the zipper teeth with a number two pencil loosens the zipper teeth. The naturally occurring grease in the graphite of the pencil gives the zipper movement. Lip balm and candle wax also work to loosen a zipper.