The weapons that you find on the battlefield in CoD Modern Warfare 2 are generated randomly, so be sure to keep your eyes open for good stuff. If you see a weapon/attachment combo that you really like, you should grab it because it not show up again. It’s a good idea to grow accustomed to all of the weapons.
Be aware of bullet penetration. Just because something looks like good cover doesn’t mean that it is. You’d be surprised what the larger caliber bullets can rip through, like your skull. Over time, you’ll come to learn what makes for good cover and what doesn’t.
Enemies are smart and react accordingly to what is happening. You can use this to your advantage. If you’re being overwhelmed with enemy fire, just fire back, whether or not you have a shot. This will force enemies to take cover and go prone, which will buy you a few seconds to catch your breath and get your bearings.
What’s dead isn’t always dead. Multiplayer veterans are no doubt aware of Last Stand, a perk that isn’t exclusive to multiplayer. A.I. enemies in the single player campaign can sometimes fall into last stand, so be sure that when you put someone down, they’re down for good. On the harder difficulties, all it takes is a few shots to kill you.
When you take a lot of damage in Modern Warfare 2, your screen will blur and become covered in blood. When this happens, you’re likely a bullet away from a dirt nap. Get to cover immediately. This usually involves going prone and rapidly crawling away. Do whatever it takes, just go in the opposite direction of the bullets.
Friendly fire hurts, so don’t do it. If you’re close enough to your allies that your HUD recongnizes them as friendly, you won’t be able to fire. But you can be far enough away where your HUD can’t identify them, and in that case you will be able to open fire and kill them. You can kill non-essential allies, though it isn’t smart. Killing essential characters will automatically end the mission.
Article from Gamersyndrome.com