Found some really helpful tips in this blog post “How to be a better Halo 3 player.” This guy supposedly works for Microsoft and sounds like he knows his Halo 3 strategy inside and out. Here a couple pearls from the post:

AIM where people might appear, if you’re heading up a ramp you shouldn’t be aiming at the ramp. Aim chest-level at edge of the doorway at the top of the ramp where someone might appear.

How do you use the battle rifle successfully? Body, Body, Body, Head… Put the first few shots into the body and then move to the head once the shields are down. Always be strafing, and use your strafing to make minute adjustments to your aim.

What controller sensitively should I get used to? 10? Probably no more than 5 for accuracy. Some people like high sensitivity so they can turn and respond to fire quickly, but that just means they suck at map awareness and they’re constantly being surprised.