Porcelain and ceramic tiles are clay-based and generally covered with a clear glaze to make them shiny, scratch-resistant and easy to clean. That glaze can start to dull and wear over the years. You can't realistically re-glaze the tiles the way it was put on originally (because it's baked on), but a workable alternative is to apply polyurethane gloss, the same stuff that's brushed onto wood floors to make them shine. The slick surface of the tile has to be de-glossed as much as possible for this to work.
Things You'll Need
Vibrating hand sander
Aluminum oxide sandpaper (medium abrasion)
Run your vibrating hand sander and aluminum oxide sandpaper over the surface of the porcelain tiles. Move the sander slowly and press firmly to dull what's left of the old glaze as much as possible. Don't sand the grout lines.
Wipe off the surface of all dust, using a dry rag.
Brush polyurethane over each tile, using a finishing brush. Brush it on in thin, even strokes, all going in the same direction. Take care to keep bubbles from forming. Don't gloss over the grout lines, just the tile surface. Let the polyurethane dry for 24 hours.
Buff the surface by hand with fine sandpaper, getting the shine dull so the next coat will adhere. Brush off the dust.
Apply a second layer of polyurethane in the same was as you did the first. Let it dry, sand it by hand and wipe off the dust.
Apply a third and final layer of polyurethane. Let it dry.