Full power-up
While playing, pause then press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.
Continue playing
Quickly press Down, Up, B, A, B, A, B, A, B, A after you've lost your last ship.
30 lives, player one
At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.
30 lives, player two
At the title screen, press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Select, Start.