A stuck zipper isn't just annoying -- if it's cold outside and your coat won't zip up, it can be downright dangerous. A zipper won't slide well if it's rusted or if debris has gotten trapped inside it. As long as the zipper isn't broken though, you probably don't need a tailor to fix it. Soap, pencils and other basic household items lubricate stiff zippers, making it much easier to pull them up and down.
Lay the zippered garment down flat on a hard surface, such as a table or counter.
Soak a cotton ball in white vinegar. Rub the front of the zipper from top to bottom with the cotton ball, then repeat on the other side. The vinegar dissolves small pieces of debris that can cause the zipper to lock up. If the garment isn't colorfast, try not to get vinegar on the fabric itself.
Sharpen a graphite pencil, then rub the tip of the pencil up and down the length of the zipper. Repeat this process on the back side of the zipper. Graphite is a dry lubricant that allows zippers to slide more easily.
Rub some bar soap, candle wax or crayon on both sides of the zipper. The waxiness lubricates the zipper, unsticking it. Almost any waxy material will do the job if you don't have these items on hand.
Wipe off any excess wax, graphite dust or other debris with a cloth. Attempt to slide the zipper up and down. If the zipper doesn't slide easily, coat it with some additional graphite or waxy material, then try again.