While elementary, it's crucial for any self-respecting fashionista to know how to unstick a zipper. Just so that I can be sure you're "in the know" I'm sharing a few of the easy tricks to unstick a stuck zipper.
For history buffs, the anatomy of a zipper.
1. Candle Wax - rub it over the zipper tracks.
2. WD-40 - spray it on a paper towel and carefully rub it over the zipper. Be cautious about getting it on the surrounding fabric...it could leave oily marks.
3. Lead - rub a pencil over the zipper. I personally don't use this one since inevitably I get lead dust all over my clothes.
4. Soap - use a bar of soap or dishwashing liquid to work the zipper lose. Especiallly good if you have a thread stuck in the zipper, as it'll help loosen it too.
5. Take it to a tailor - if it's a favorite piece, take it to a tailor and have him/her replace the offending zipper. I do this for more expensive cocktail dresses that always seem to lose the zipper pull.