Have 6 Continues
Hold Up + L + R, and press Start at the Konami Screen.

Infinite Napalm
Walk through the first mission without shooting or hitting any targets. Your score should be 0 points. Defeat the Boss without shooting its side gun turrets. Your score should be 2800 points. When the asteroid level begins, press X to change weapons until the unlimited napalm gun is selected. Survive this mission to keep the unlimited napalm for the remainder of the game.

Note: When doing this, it cannot be powered up.