Know your weapon damage and time your shots correctly. Some weapons are better for kill-stealing than others (high damage spikes), but it works for most of them. Concentrate on enemies that give a lot of points and don't take too long to kill. Leave the bosses for your teammates or for last, but be aware of their armor bar so that you can try to make the kill-shot. Headshots = higher damage spikes, don't be a scrub.
Many players focus their fire on big targets, kill the small stuff while they are busy. Before reviving a teammate kill everything in the vicinity to be safe. Did he bleed out while you were doing that? Aww, too bad.
Rocket spawns 0.5 seconds faster than your teammates. Nothing is more frustrating than nuking a juicy spawn and seeing somebody else's name pop up in the kill feed. Only rocket spawns during wave 1-10. That way when someone else starts nuking spawns on extraction, desperately trying to catch up to your massive score, you can look down on them with haughty derision.
If somebody is close to catching up with you on extraction wave, kill enemies away from him. That way the chances of him getting overrun are higher. 1-2 seconds before extraction, paint every enemy you see with your gun and powers. They will die when you extract and you will get the points without actually killing them.
Or alternatively, hang back once in a while and let somebody else outscore you. It'll make them feel good about themselves and you can have a relaxing game before putting on your tryhard pants again. Playing without pants is more fun anyway.