At the Title Screen press Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start.
After you beat the game, the credits roll by and you start over with the remaining lives you had. You will also start with the gun you had. This time around, there will be more enemies. They will also shoot a lot more, and some enemies may have more hit points.
During the ending, before the closing credits starts rolling, press and hold the Select and Start and keep on holding until the credits are over. If done correctly, you will receive a message from Red Falcon, vowing his revenge.
When you enter the Base 1 or 2 levels, you can shoot high while ducking. Hold Up on Controller 2 while Controller 1 is ducking. You will be ducking but your bullets will appear to go high. This may be hard to pull off yourself, so help is recommended.
At the title screen, press and hold the A & B buttons simultaneously and push Start.
Enter the "Konami Code" (Up, Up, Down, Down, Left, Right, Left, Right, B, A) at the title screen while it's scrolling. If done correctly when you start a game, you'l have 30 lives instead of 3.
If you die in a two player game, hold down A and B simultaneously