spit, potato, and special sprays, all work on the same principle. They reduce surface tension, which encorages water to form a film over the surface, instead of droplets, which scatters light less than droplets do. Spit works okay, on glass, which has a low surface tension to begin with. It doesn't work well on modern plastics, which has higher surface tensions. It also washes off quikly.

I've used a bunch of these, and haven't found one that's spectacular. There's a lot of variation based on the surface they're going on, too, so what works great on one visor might not work as well on another.