Beeswax (any surfie shop) on the inner teeth, old candles work too
. The outer teeth need to be kept clean, lubed and free of salt. Scubapro make a great brush-on lube, it's a liquid wax, any dive shop should stock. I'm sure lots of products, many cheaper, would work but you don't want to attack the rubber around the zip with oil based waxes etc. Most zips get stiff and then f'ed when forced in a hurry. I'd think a mild acid-based cleaner would be good on an old zip with corrosion, followed by a good rinse and lots of lube
. Obviously cleaning after use is a must, unless you're in fresh water.