The best ranking you can get is "Galaxy Defender," where you'll be told that your fresh new MIB suit will be ready for you next Wednesday.
The tips that I've found useful are these:
1) The gun is automatic. That is, just hold down the trigger and it will keep firing. It took me forever to figure that one out, but my finger thanked me once I did.
2) At the start of the ride, in the "target shooting" area, pick one of the aliens and shoot it as much as you can, using tip #1. This way you don't waste time aiming on a different target after each shot. My max score leaving this area was ~41,000, but I'm sure that can easily be beaten.
3) My friend swears by this rule, but I don't know if it really helps or not. He says to concentrate on the smaller aliens hiding up in the windows and such.
4) Many of the aliens later on can be shot several times, like the alien in the window with the one big eye.
5) If you are on the right side track, you can still hit some of the left side's aliens, like the ones in the trees just before the scan.
6) The fusion exhaust port. After a while, the car opposite you will stop spinning when its port is hit. That doesn't mean you can't still score off of it... Using tip #1 you can hit it 5 or 6 times in a row quite easily.
7) Push the red button at the end.
My personal best is 735,375, but I'm pretty sure I remember someone posting here a while back a score higher than that (and I don't mean 999,999 due to the ride stopping).
Good luck!