Increase your Field of View
Field of View or FOV is another crucial setting that nearly all of the top gamers change. This just isn't something that players change in Call of Duty, but in almost every competitive video game.
Your Field of View determines how zoomed in your screen is or how much your screen is zoomed out. The lower the number, the more zoomed in your screen will be, the higher the number, the more zoomed out your screen will be.
I believe the default setting for Call of Duty: Modern Warfare 3 sets your Field of View at 80. Changing it to 100+ is recommend. Most pros would tell you that between 100-110 is the sweet spot, but this is something that you might have to experiment with yourself. However, anything greater than 80 is a bonus.
The reason why a higher Field of View is recommended is because you get to see more of the map in front of you. Just by increasing your FOV by 10 or 20 points, you have the chance to see enemies in your peripherals where you might not have seen them before by using the default 80 setting because your Field of View was increased.