For a while I've been struggling with a really nice cycling jacket that became difficult to zip up: the teeth would fail to mesh together, and would pull apart. Well, after a bit of research on the web I couldn't find a definitive solution to this problem, so tried the only thing that was available to me: squeeze the zip puller with pliers, to make it run tighter against the teeth.
Lo and behold, it worked! Use long-nose pliers to get a reasonably even force on the zipper, and squeeze them gently on one side at a time to narrow the gap a little bit. You really don't want to narrow the gap much; just enough to get a better fix on each zipper line. Simple!