Enter the following at the "Press Start" screen:

Master code:
, , L2, R2.

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

Big heads:
, , Up, Down.

Small heads:
, , Down, Up.

Random sized boxers:
, Left, Up, .

Flat mode:
Down, Up, , .

View credits:
, , , .

Thanks to Revolution readers Kyle and Boondoggy!