You need to break the glaze on the tile first.
Put some insulation tape over the spot where you want to drill. This will stop the drill from slipping, then place the drill bit against the spot and twist left and right a few times to break the glaze. Now use the drill to drill through. Dont use the hammer on the drill until you are through the tile. Or buy a drill bit especially designed for the job...Spear bit I think they're called.
Alex
February 2008