Mirror Room Star
In order for this to work you will need Luigi. Play as him and go to the third story of the castle.
Then enter the first door to the North East. It should have a star without a number.
When you enter it, there should be power flowers on the pedestals. Grab one and walk through the mirror.
Then go to the reflection version of the door that you came in through. If you enter it then you'll be in an all white room with a power star.
More tricks, tips and cheats for this game are right here - Super Mario 64 DS cheats
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