Enter the following at the "Press Start" screen:

Master code:
Square, Circle, L2, R2.

All created boxer items:
L1, R1, X, X, Triangle, X.

Big heads:
Square, Circle, Up, Down.

Small heads:
Square, Circle, Down, Up.

Random sized boxers:
Square, Left, Up, Triangle.

Flat mode:
Down, Up, Circle, Square.

View credits:
X, Triangle, Square, Circle.

Thanks to Revolution readers Kyle and Boondoggy!