Killerwatt

Steer your ship such that a spawning point for white aliens is *exactly* at the edge of the screen, so you can still see just a little of the alien that spawns. Shoot him, and see if he reappears (if he doesn't you need to move slightly farther away). If you get exactly the right position, he'll keep spawning but you can still hit him. Then just point your nose at him, put down the joystick and put a weight on the fire button, and your score will keep rising and rising for as long as you keep the Commodore on. (Careful: no inbuilt fans in the Commodore so don't overdo it.)