We have to replace most of our jackets through the zips failing then any other wear and tear. The cost of individual replacement zips, is frequently almost the same price as replacing the jackets!
Washing the zip mechanism in hot/warm water regularly and then treat with wax. Ordinary bees wax is great, dont bother going to a dive shop to get this, as you will pay twice the price.
Still dont understand why many of the big marine brands, still insist in using aluminium zip pulls, as they always corrode evertually, leaving the plastic zip sides intact.