I'm no whiz (my best is 700,000-something) but I think I've gleaned the theory from many GW threads -
The key to raising your score is lasting as long as possible with one life. 5 mins on one life is way more productive than 10 mins over 5 lives because of the multipliers. So, you may as well restart if you die before 100,000 or so, and use your bombs to stay alive when you have a good multiplier streak going, even if you have lots of spare lives built up. Apart from that, just focus on clearing an area enough to move through it, rather than trying to clear whole waves of enemies, or clearing the whole screen. Manage the gravity wells carefully - you'll need to leave some of them around to hoover up enemies once it gets hectic, and taking them out just before they explode is a great way to raise your score, but I find more than 4 onscreen means you quickly run out of space to maneuver. That's about it - everything else is just twitch and rhythm.
