Ceramic tiles are glazed while they're being manufactured, with the glaze baked right onto the tile. You can't really recreate this process once the tile starts to lose its shine, but there is an alternative. You can coat the tile with polyurethane, the same stuff that is used to finish wood floors. Whether the tile is on a floor or a wall, this will give it a new, durable shine. You can use either oil- or water-based polyurethane, but if you're trying to get the job done in a hurry, water-based is your best bet because it dries more quickly.
Buff the surface of the tiles with a vibrating sander and 80-grit sandpaper, dulling whatever shine remains as much as possible.
Put on your rubber gloves and mix 1 cup TSP and 1 gallon of warm water in a bucket. Scrub the tiles with the mixture and a stiff scrub brush. Rinse with a sponge and clean water. Let the tiles dry.
Brush high-gloss or gloss polyurethane over the tiles, starting at one end and working to the other. Brush it on in a smooth, even coat, in one direction. If you're finishing a whole floor, brush around the edges, feathering the brush strokes out gently, and coat the middle with a finish applicator. Let the polyurethane dry for at least four hours.
Buff the dried polyurethane with 260-grit sandpaper--by hand, not with your sander--just enough to remove the shine so the next coat will stick. Vacuum and wipe the tiles with dampened tack cloths to remove all dust. Wait for the tile to dry after wiping it clean.
Apply a second coat of polyurethane. Let it dry, sand it and remove all traces of dust. Brush on a final coat and let it set for two days.
Ventilate the room when applying the polyurethane to avoid inhaling fumes.