2-3 sensitivity it gives you better accuracy than higher sensitivity, but if you get shot from a place other than in front of you, you might not be able to react in time. If you use a higher sensitivity, you will be able to turn faster, giving you an advantage, but your accuracy might go down as well.
 So many people do this without thinking. Take your time and go round the corner already aiming down sights. You'll gain a massive advantage. If in doubt, then chuck a flashbang or a concussion grenade.
 Practice one on one with a person of similar skill level, or higher if you want. Practice against them in a large map. Switch between this and normal play every couple game sessions. You will get much better at aiming.
 If your using Ghost Pro, you can really help out your team by taking out a chopper without much self-risk. Keep a class slot free just to take down all choppers.
Find a comfortable sensitivity.
Don't sprint around corners.
Practice aiming.
Shoot down choppers.
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