Tile lippage can be stopped by making sure that the surface is as flat as possible before any installation of the tiles begins. Use a correct trowel and thinset thickness for the tile and carefully place each tile to help reduce the risk of lippage. Sometimes it is required to lift the tiles and add more thinset/mortar as required. Lastly, use of a tile leveling system can help fine tune your tile levels and keep them inplace during the thinset curing process.