Melt it in the top of a double boiler - white chocolate burns so easily, and once it starts to go, it's ruined. We have scorched it in a microwave oven, which works great for dark and milk chocolate; but you can't seem to watch white chocolate closely enough in a microwave.
If you don't have an official double boiler, suspend a metal or heat-proof bowl across the top of a pan with a little simmering water in it. Make sure the bottom of the bowl or pan doesn't come in contact with the water below. Put the chocolate in the bowl and it will melt safely. Putting a lid on top will speed the process. Be careful not to let any water or steam get into the chocolate, as it will cause the chocolate to seize up.