Round Select:

At the "Quit/Previews" option on the option menu, hold the X button and press B, L, and A. Then let go of the X button and press B, L, A, B, L, A. You'll hear a confirmation sound and the window will display a round select.

Free Flight Zone:

To access an endless zone free of enemies to practice flying around in: Start the game, and pause it with the P button. With Play Game highlighted, press B, A, C, A, B, A, L. Press the L and R buttons simultaneously, and X, X. A skull should appear in the window to the right. Then press L, A, B, L, A, B, X, X, X.

99 Ship Code:

Proceed through the Free Flight Zone code until the skull appears in the window. Press P. A much bigger skull should show. Press A, A, B, B, C, C, L, L, R, R, P. This should restart the game from the beginning with 99 ships.