A couple of things that work for me. You may be different...
The larger the screen, the better. Playing on small screens makes missing incoming fire harder for me. For flat screens that means 50" plasma, CRT for me is 36".
Another thing, and this applies to all fast past games in general; you have to learn to play relaxed. You may think you are relaxed when you might not be. As you're playing, think about your arms, shoulders, hands. Are you able to relax them more than they state they are in when you think about it? If so, you aren't playing relaxed.
Back when I was playing Quake live, an old clan buddy taught me about this. My gameplay improved tremendously.
It's not easy to do at first, it takes a bit of training. But once you get it down, your performance will show it.
Also with a lot of vertical shooters, you almost never want to be on the edges of the screen. Rather than let bullets push you to the side where you inevitably have to walk up the screen to miss incoming fire, you're better off looking for openings and crossing in front of incoming rounds to maintain distance from the edges of the screen. Being able to move in all directions at all times keeps your options open, whereas staying at the bottom or to the side gets you trapped.
Also, when you dodge large volleys of incoming fire, you want to just barely miss it most of the time. With most shooters, the enemy shoots directly at you. If a volley of fire comes and you move away in an extreme manner, the next volley comes your way, and then you've filled the screen with enemy fire. On the other hand, if you make small moves, you can keep enemy fire more concentrated which translates into more open space for you.
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