I think the biggest thing is to not milk the stage boss. If you clear him (and the preceding waves) fast enough, there is an extra hidden Z bonus on the island on the right side of the screen shortly after. And there is another Orb thing, so if you shoot that and all four smaller orbs it's another bonus.
Those brownish robots that run away from you are each worth a lot of points. You can stop shooting for a moment to let them get close, and then try to take them all out before they can escape.
Kill the snake from the middle once it completely closes around you.
Destroy both those eye-opening ships at the same time.
Shoot all four mini-orbs that come out of the big orb (at the beginning of the level and again at the very end after the boss).
You have a bit of invincibilty after picking up a powerup, so there are a few places were you can use that to fire directly on top of an enemy.
You can also use your booster thrust to damage enemies as you change speed. There's one ship that comes up from behind where this is useful (I could never get the hang of this... maybe with a 3-button pad...)
And remember, the next wave of enemies doesn't appear until the current one is gone, so kill everything fast and get more waves in. All while shooting everything in the environment.
I'm not sure of the maximum possible score, but it's definitely over 700k based on the contest last year!