What can be done to improve tile appearance when surface glazing has already been damaged?

I am assuming you are referring to glaze that has been damaged by improper cleaning methods. While the glaze can't be repaired, you can use a silicone tile/grout sealer to hide some of the damage and make the tile look shiny. The sealer will probably only last 1-2 years. It would of course be wise to make sure the tile is clean before sealing.