Unlock Stage Select:
During the Intro sequence when the planes fly across the screen press Up, Down-C, Left, Right-C, Down, Up-C, Right, Left-C, L, R, Z.

Unlock Extra Pilot and Plane:
At the Title Screen press Left-C, Down-C, right-C, Up-C, Left-C, Right-C, Down-C.

Unlock Mao Mao:
At the Main menu Screen press Left-C, Bottom-C, Right-C, Top-C, Left-C, Right-C, Bottom-C.