c0rps3K1lla008 at High Impact Halo has posted a guide to maximizing your campaign score on Halo 3. It makes a good companion piece to the guide on skulls posted at Bungie.net. This one line, however, is genius and bears quoting:

Turning on Catch for Floodgate, Cortana, and Halo gives a free 1.5x multiplier! Why? Because the Flood do not throw grenades!

You might also consider adding Famine. Since a lot of Flood forms are melee only, this lessens the impact on gameplay on those levels.

Check out his guide for more useful tips. Now excuse me... I have to go back and play a couple levels...