Press T to display the message prompt, then type one of the following codes:

spindive		Can't be hit 
paperairplane		Paper airplane
damn that corner	Can't crash    
upside down		Fly upside down on Autolevel
big gulp		Refuel
youre here forever	Invincibility
chiliburger		Repair damage
you got what i need 	Unlimited ammunition

                             
                              

Thanks to Revolution reader RTM!