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: RRJABBA. When you're finished, select "End" and hit L again. Now enter your name the usual way.

Start a race and press START to pause it. Now press D-Pad left, down, right, up -- and you will be invincible.