Rapid-fire lasers:

Press Up(2), Down(2), Left, Right, A(2), B(2), C(2) after the 
Starblade logo appears on the title screen and after the animated 
background has stopped. The lasers will fire very fast, destroying 
tough targets much more quickly. Hold the fire button down to get 
rapid fire. 

Unlimited continues:

Press Up, Right, Down, Left, A, B, C, Up, Left, Down, Right after 
the Starblade logo appears on the title screen and after the 
animated background has stopped. The "Continues Left" counter will 
read "FREE PLAY" instead of a number. Note that this is only 
visible after the first ship has been lost and the game is at the 
"Continue" screen. 

Multiple codes:

It is possible to have both of these codes active simultaneously. 
Wait until the title screen appears for the second time before 
entering the second code.