Lots of practice. I used to be very good myself. I played Halo and often I'd just run around and try to hit as many targets as I could. Pick something in the map and just try to hit it, doing all sorts of things like jumping, running. Stop, turn and shoot. Stuff like that. I quit playing shooters and started playing more rpg type games and especially WoW. Now, I'm no longer a very good fps gamer. But, I'm trying to get better.