Select an empty game save (to erase, press C-Up), the hold the Z button while scrolling through the letters on the name entry screen. To enter a letter, hit the L Button. Enter the word: RRTHEBEAST. When you're finished, select "End" and hit L again. Now enter your name the usual way.

Pause the race during gameplay and enter D-Pad left, down, right, up to activate mirror mode.