To even the playing field and have max stats for all boxers, go to the main menu and enter this code: LEFT, UP, UP, DOWN, RIGHT, L while holding the R button (thanks to Mike).

Screw the work out. On the main menu hold R then press RIGHT, DOWN, DOWN, UP, LEFT, L.

Unlock all the boxers, statues, and arenas by doing this at the main menu: hold R, press UP, DOWN, DOWN, LEFT, LEFT, L. Submitted by Drederick Tatum

On the main menu, hold R then press RIGHT, RIGHT, LEFT, LEFT, UP, L. While fighting in movie mode, get an instant win by pressing BLACK + WHITE.

These codes increase punch damage and speed. Enter them in the main menu.

Damage - hold R, press RIGHT, DOWN, LEFT, UP, LEFT, L.
Speed - hold R, press DOWN, LEFT, DOWN, UP, RIGHT, L.

To unlock all boxers (except Rocky Statue and Mickey) and all arenas, go to the main menu and press these buttons: RIGHT, DOWN, UP, LEFT, UP, L while holding the R button (thanks to Mike).

Beat the Bronze version of the Knockout Tournament.

Beat Movie Mode while playing through on the Contender setting.

Beat Movie Mode while playing through on the Champ setting.

Beat the Silver version of the Knockout Tournament