Hidden music track:
Press Select, Up, Down, Left, Right, Up, Down, Left, Right, Up, Down, Left, Right at 
the main menu.

Hidden skin 1:
Press Select, Left, Right, Left, Right, Left, Right, Left, Right, Left, Right, Left, Right 
at the main menu.

Hidden skin 2:
Press Select, Left(2), Right(2), Left(2), Right(2), Left(2), Right(2) at the main menu.

Hidden skin 3:
Press Select, Left(3), Right(3), Left(3), Right(3) at the main menu. 

Hidden skin 4:
Press Select, Left(4), Right(4), Left(4) at the main menu.