In the first room with the big orange robot. After you pass the robot turn your car all the way around and hit him on the back of his left hand or the inside of his right hand. It gives you something like 100,000 points each time.
You don't have to wait until you pass him. You can get 100,000 by hitting the target on the front of the hand, so start hitting it as soon as you can start shooting. (It's his left hand, so it'd be on your right). Also, on the right of the big orange robot, there is a claw that is worth 100,000. You have to look up to hit this one. I normally start hitting this while I leaving the room because I think it's easier than getting the robot's arm while leaving the room. (If you can keep hitting these two targets, you should be around 400-600,000.
When you get in the next room hit the volcano (it's only around 25,000 but I think it's the highest target in the room) Don't let go of the trigger, just keep it on the volcano and keep it down. There is a jack in the box near the end of the room with the volcano, you open it by hitting the target on the right side of it, and then on the back of the box behind the monster that goes up and down is a target worth 25,000. You can almost be to 999,999 by now, but will need to hit the bottom of Zurg's ship a couple times! This is on the right hand side after the room with the volcano. You have to hit it coming in the room and leaving it, but like i said, never let go of your trigger.
If you are still a little short, there is actually a sensor in the big projector room on the wall. It's really hard to find. I think there is one on both sides, but the one on the left is easier to find. If you look out for a blue space ship and an orange one near the end of the tunnel, one of them hovers for a second, you can see a little black dot on the wall, try hitting that I think it's around 25,000 again. Those are all the bigger ones I know, but like someone else mentioned, the secret is to never let go of the trigger!