All Styles
Press the following combintation at the 'Style Select' screen:
Left, Up, Right, Down, Left, Down, Right, Up, Left, Modifier Airs.
All Bikes
At 'Bike Select' screen Press:
Up, Left, Up, Down, Up, Right, Left, Right, Modifier Trick Button.

Slim Jim
On the 'Rider Select' screen press:
DOWN, DOWN, LEFT, RIGHT, UP, UP, Modifier Airs.
Unlock Dave Mirra Video:
Beat the 'Pro Quest' mode as Dave Mirra to unlock the Dave Mirra video.