Invincibility
Press [Up, Right, Down, Left, Up] four times at the copyright screen while the letters are falling from the top of the screen. Then press Start to enable invincibility. Alternatively, press Up, Right, Down, Left, Up, Left, Down, Right, Up.
Level skip
Press Up, A, B on controller two.
Nine lives, all weapons
Press Start and wait until the title screen appears. Then, hold A + Down and press Start to begin game play.
Barrel roll
Hold Select and press the D-pad while flying to perform a barrel roll in that direction.