At the Main Menu, hold R and enter the following:

Double punch damage:
Right, Down, Left, Up, Left, L.

Double speed boxing:
Down, Left, Down, Up, Right, L

All default boxers and arenas:
Right, Down, Up, Left, Up, L

All default boxers, arenas, and Rocky Statue:
Right, Right, Right, Left, Right, L

All default boxers, arenas, Rocky Statue, and Mickey:
Up, Down, Down, Left, Left, L

Full stats in tournament and exhibition modes:
Left, Up, Up, Down, Right, L

Full stats in movie mode:
Right, Down, Down, Up, Left, L

Win fight in movie mode:
Right, Right, Left, Left, Up, L1. Now press Black + White during a fight in movie mode to automatically win.

Thanks to Revolution readers Martin Brodeur and Rocko!