you have to find a dead flat surface and some coarse sand paper, remove the pads, and rub them up and down the sand paper until the surface is fresh and rough all over (no longer polished). It is also worth giving the discs a good rub down with the sandpaper to get rid of any shiny spots. Always remember that the pads might need to be bedded in a bit but it should cure any noises for a while. (Maybe not long enough to make it worth all the effort though).
Matt.