Go to the main menu (The menu that shows when you don't have a game in progress.) and select Personal Info. Go to the Manage Users option. In there, select Change Name. There, you can enter these codes. After you enter them, you should see "Cheat On" in red.
Note: the "_" is a space.

Password	Effect
FORSIX	24 Lives.
I_GUNZ	Unlock a different gun.
NO_ID_	Unlock more backgrounds.
_PWR_B	Unlock Powerball.
ME_PAZ	Unlock the Laser gun.