I've looked at this thread a few times and have decided to give the 3 tips that wll have you to 1,000,000 in no time.
1. Fire in front of you, but not only that, strafe the fire as if you were the Terminator. This way you control an area in front of you.
2. Circle. The reason I state this is because the "enemy ships" as I call them (the red things that actually fire) travel in a straight line. This way you can avoid them untll you shoot them or they get sucked into a black hole.
3. Use bombs often. The multiplier only stays with the life you started it with. This means that the longer you stay alive, the easier it will be to earn a high score. You will find that earning enough points to get new bombs will be easier when you have a high multiplier. Using bombs, as oppose to dying and wasting a ship, makes all the achievements easier to accomplish.