A technique I learned a while back that improved my k/d and got those sniper headshot kills is starting the match at the highest sensitivity 10. After about half way through the first match I drop it down to about 5 or 6 and the difference is significant. I guess your eyes get used to how fast everything was moving before that once you bring down the speed it makes things so much easier. It's almost like that movie Wanted lol. Try it out, this technique actually works with any FPS. Do that only for your first match and once you adjust yourself you'll be fine.