Here's a post of mines that I posted before on a similar topic.
•Always (x50) reload! I've suffered quite a few embarrassing deaths trying to take out 3 people with only 5 bullets left in the mag. Even if you are using an LMG, just do it.Trust me, your K/D will thank you. Which leads me to...
•I know you've heard Gaz say it, but switching weapons is always faster than reloading.
•A wise man once told me it's quicker to stab than shoot.
•Don't concentrate on upping your stats or gaining a devastating Scorestreak. You'll freak out at every death, lose focus, and take things way too seriously..and that's just no fun. It's just a game, so treat it as such.
•If you start to go into a succession of bad games, than take a break and play/do something else for a few hours. Come back when you're in a good mood and able to focus. Your teammates will thank you.
•Every so often, do a quick glance behind you. There are some sneaky players out there who get a kick out out of running around stabbing you in the back (literally)..
• In team games, if you suddenly see 3 skulls in quick succession (indicating teammate deaths), um...it may not be a good idea to go there. If you do, be extremely cautious.
•Check the scoreboard every few minutes. If you see a player with a high K/D in the match, be prepared to face a nasty Scorestreak soon...
•Lastly, constantly eye the the bloody mini-map. If you see a red dot behind you, than that means an enemy player is actually behind you. This seems like a no brainer, but it surprises me how many players ignore this simple fact.
•Stay FAR away from flammable explosive objects (such as cars). They have this awful tendency to explode at the most inconvenient of times (such as when you are happily trotting by). I don't know if there is an employee in some warehouse watching the map with the trigger in hand or what, but stay away.