plexus is the shizzle. Plexus and a microfiber cloth will keep the shield clean, streak free, and fog resistant for years and years.
When you get a scratch though, if you know what you're doing, Novus makes some acrylic products that can buff them out. Only good for the outside though, dont ever get the stuff on the inside and forget it on mirrored shields. If you dont quite have the technique down, then buying a new shield isnt a bad idea.