Automatic win:
Select an opponent, then hold L1 + L2 + R1 + R2 and press X at the confirmation screen (the last screen before the fight). Instead of fighting your opponent, the screen will turn black and display the message "Now Loading". Press X to return to the profile screen and win the bout without actually fighting.

Change attributes:
Press Start + Select during game play to change your boxer's attributes.

Hidden fighters:
Successfully use a single boxer to defeat all the hidden fighters (Jackal, Dragon, John Summers, Gonzo, and Panther) when challenged. Save that boxer, then enter exhibition mode. Press Triangle to display the load character screen. Press Triangle to exit the screen without loading. Jackal, Dragon, John Summers, Gonzo, and Panther will now be selectable.