On the screen where you choose how many players you want press: up,up,down,down,left,right, left,right,b,a,b,a,start. That should give you 30 lives instead of three!
Cheat
At the title screen, quickly press Up, Up, Down, Down, Left, Right, Left, Right, B, A, B, A. You will then start with 30 lives. The really cool part is that after a continue, these lives come back again!
Hard Difficulty
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.
Secret Ending (Japanese Version)
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.
Shoot while ducking
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.
Sound Mode (JPN Only)
At the title screen, press and hold the A & B buttons simultaneously and push Start.
Stage Select (Japanese version only)
At the title screen, press and hold Up, Left and the A button. While holding these buttons, press Start and immediately hold Select. If done correctly, the stage select screen should appear.
Steal life from partner
If you die in a two player game, hold down A and B simultaneously.
30 Lives...