If you are over correcting fairly consistently I'd try turning down your sensitivity a notch or two and playing with it lower for a while. Your close combat game will suffer from that, as you can't turn as quickly but it should help improve your aim. Once you start to get used to it you can work your way back to a higher sensitivity. When your target is moving you are going to want to aim just in front of them, unless they are pretty close to you. Try keeping your cross hairs at head level at all times. I'm almost always looking slightly upwards. The less you have to move the crashairs when someone comes onto your screen the better chance you have at killing them. ALWAYS ditch the assault rifle in favor of a battle rifle in games when you don't start out with one. Use the BR whenever you have the chance and just work on headshots. As soon as you can get decent with it the BR will beat an AR almost everytime unless it is at extreme close range. It's hard to explain a lot of this because I've just been doing it for as long as I've been playing Halo, so your best bet too will be to watch film clips in the theater. If you really get your ass handed to you and it seems the other team is getting headshots every time, go back in and watch what the other players do, and compare it to yourself. Theater is a great way to get better, just study and observe better players and you will start to pick up a few things.