At the main menu, hold L + C-Right + C-Down, then press A. This will display the cheat code screen, where you need to enter the following codes:

Effect			Code  
 
Stunt Bonus		SHOWOFF
Invisible Riders	INVISRIDER
Big Heads		BLAHBLAH
Mirror Mode		YADAYADA
Unlock All Stunts	TRICKSTER 
Night Mode		MIDNIGHT