Some common tips for sniper that I didn't see mentioned:
* The silent pistol is your friend. Enemies spin in confusion if you're firing even from close range. If you're hidden reasonably, it'll take them too long to spot you. Challenge is getting close enough in the first place.
* If you're in a tight alley with no cover, carry the claymore as a primary weapon. Someone appears in front of you, just drop the claymore. They'll go pop, and you escape with whatever damage they did spraying bullets at you.
* Fire a big-caliber rifle round at their torso, then switch to pistol and fire away with that to finish off their last five percent of health.
Having said all this, I will point out that I'm no scope-meister. I love being up on a hill spotting and sniping, but I'm not great at avoiding unwanted attention from choppers.