1.) Try and stick with the default controls, trust me they work best. But if you're really finding it hard to adjust even after some time, go with alternate 2 (closer to COD4).
2.) Try and stick with default sensitivity, it really is best for pin point accuracy despite being slow. Otherwise up the sensitivity a few notches (on both X and Y), then when you are more used to the aiming, drop it back to default.
3.) Be sure to use the entire range of the analogue sticks. Don't just harsh press left, right, up etc in one direction the full way at the same strength/force all the time. Be elegant with how you aim. Use gentle touches, taps and so on where necessary, and longer/harder pulls/pushes on the stick in others. The analogues aren't just a directional guideline in this game, they are a strength gauge. The harder/longer/stronger you pull in any direction, the quicker and more forcefully they will go that way. Sometimes delicate touches are all that is necessary for precision shots.
4.) Take your time. Don't panic and rush to get off some shots. It's better to take your time to find the right line with the shot for a quicker kill and/or to get that super rewarding and efficient head shot.
5.) Burst fire every time. The longer you hold the trigger, the more recoil and wasted bullets you face. Use quick bursts, that way you retain accuracy and your bullets are far more lethal.
6.) Pretty obvious, but stick to burst fire from the hip for close/medium quarters, and iron sights for anything further. Again, stick to using burst fire otherwise you will end up spraying all over the place.