Only tips I can give you are the ones I use.
- Pay attention to where your team is, it'll help you identify where the enemy team is more than likely at. Helps if you are using the Specialist Kill Streak or going for that high Assault Kill Streak.
- Find what perks and guns suit you the best. I personally use Scavenger, Hardline, and Stalker Pro on all my classes. Mainly for the ammo, every kill streak is one less, and the ability to aim around corners faster. Oh, and those annoying claymores / bouncing betty's.
- Take your time going around corners and almost always aim around. Try and be a few feet back and just take the wide approach, it'll help you identify people easier.
- When ADSing (aiming down sights), tap your left trigger a lot. I helps keep your sights from moving because of breathing. If you don't know what I understand, ADS for a while and the sight will start to rotate.
- Burst fire and never hold down your firing trigger, unless you are hip firing with a weapon.
- Start playing Infected for the first few games of the day, to get back into the groove of things. It'll help you with accuracy and rapid target accusation.
- If you want to get better at aiming, do the above. Or, if you have ten dollars laying around, buy some Kontrol Freeks online. They helped my accuracy go from 17-18% to 21-25%. They are just a little joystick extender, tournament legal, and make gaming more comfortable. (They are a pain in the butt for PS3 though.)
Those are just some tips. Hope they helped.