During gameplay pause the game and press Up, Up, Down, Down, L, R, L, R, B, A.
Unlock 30 Extra Ships:
At the 'Title' screen hold L and press A, A, A, Start.
Unlock Arcade Mode:
At the 'Options' menu highlight the Game Level and rapidly tap the A Button until the level turns into 'ARCADE'.
Unlock Extra Credits:
Rapidly press 'X' at the 'Title' screen as many times as you can and press 'Start' before before the screen fades out.
Unlock Random Weapon Select:
Press Right on the D-pad to enter 'Edit' mode at the 'Weapon Select' screen and then press X, Y, X, X, Y, Y, X, Y.
Unlock System Reset:
Pause the game and press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.
Hold L + R and the 'Start' button and then press 'Select'.