Display the console window during game play by pressing ~ (or [Shift] + 3 or
hold [Alt] and press [Keypad 2] [Keypad 1]). Enter one of the following codes
and press Enter.

Unlock in-game T-Shirts:

Enter one of the following codes at the Extras -> Network -> Enter passcode
screen. (you need a PSN account)

Randomize traits:

Enter the "Options" menu, then select "Remote Assistance". Enter "joker"

Press [Ctrl] and C button to open up cheat code entry box, then input:

Start Boost:
At the moment it says GO, press L1 for a boost.

Unlock BJ Penn:

NOTE! Activating a cheat will permanently prevent the game from
being saved and achievements from being earned.