On the main menu, press X, B, B, L, L, Y, UP, DOWN, RIGHT, UP. (Alt source, do this code on Cheats screen).
Hint: Bonus Points at Island Outskirts
To get big air and lots of bonus points at Island Outskirts, first pick your fastest car. Next, when you are racing, go to the shortcut with the movie set. Instead of exiting right away, run your car along the edge of the exit. You should go flying into the air and get lots of points, up to around 1200 points.