No enemy fire trick
On either the 1st or 2nd screen, leave one of the blue/yellow bugs, from either of the 2 left-most columns in the formation. Avoid its fire until it stops (and passes at least 3 times down the screen without firing). This takes anywhere from 5 (if both are left) to 20 minutes to perform. This bug occurs b/c the jobs of starting a new shot and moving an existing shot are handled by 2 different CPUs (there are 3 total). The problem is, each uses a different method to determine if a shot is active or not - one checks to see if X=0, the other uses a specific status byte. There are a total of 8 shot slots that can be active or not at any time. When you leave one of those bugs, it circles off and on the screen. If it shoots while off the screen, the shot is made active, but X=0; it never moves. When this happens 8 times, their shots become inactive. In a 2-player game, if either player does the trick, both will benefit.
My friends and I always wondered why this worked back when Galaga first came out. Now I know.