First and most important of all have complete confidence that the attack your going to make will be successful.

Apply the correct Perks to the correct guns, EG. stopping is not as effective as juggernaut on shotguns and the P90 etc. the possibility for a long period to fight your foe will aid in rounding doors and dodging enemy fire. Put stopping power on snipers with claymore but on a acog setup it is implied ou move more so choose special grenades or bomb squad for counter sniping.

Swap your pistol for a fallen foes gun when possible. a spare rifle or smg always helps.

keep a UAV jammer setup with a silence gun always. in a game were your team has fallen behind going invisible and sneaking behind the enemy's and disrupting them will gain you team a few precious point. Players use there radar so much that they may simply walk past you. also get away from you team mates or walk just away from them, the enemy will be drawn to them and will find you too!

Get a set of good surround sound headphones, you can hear enemies behind you and its a surprise the distance that people can be heard from.

In SnD, don't go solo or attack aggressively down sniper lines, just cause you don't see a sniper doesn't mean there isn't one there;

Always try to call out spotted enemys, it may be obvious but it may alert a teammate to the presence of a enemy to hunting them;

Overkill classes work well, try a silence rifle with a shotgun for close quarters and bomb squad to avoid claymores;

When planting claymores to block a door, point them slightly inward so the laser closest to the wall is parallel to the wall, that way your claymore will set off when the run through and actually kill them too!

Also a bit obvious but when spotted in a map like vancant or broadcast try to get the advantage by switching attack routes and legging it round the coridoors to find their back side and stab them.