Hey guys. Just thought I'd give some fantastic tips I've discovered while playing this game to help you win every single match you play. It breaks down like this, there are two main factors that affect who will win a firefight, master both and you can't lose.
1) Have the best connection - If you have the best connection, you'll probably be host, this way, you'll experience no lag at all and this will put you at a huge advantage. In the MW2 world, if you have a fast connection, it's the equivalent of having a bigger gun than the other guy. Use this to your advantage! Contact your ISP and explore your options for faster internet speeds. Then, you'll be kicking butt before you know it. IW designed the game that way, and... well, hey, I'm not here to make judgments. Use it to your advantage.
Bonus tip: Use Commando whenever possible. If you do this, you can teleport through bullets and what-not. This a great trick equivalent to an exploit that you can use to take advantage of those pesky close-quarters encounters. I highly recommend it.
2) Spot the other players first - the second factor affecting who will win in any given game of MW2 is who sees the other player first. With such high power and fast-firing weapons, if you see your opponent before they see you, you're almost guaranteed to win the fight. To this end, I recommend sticking to the out-skirts of the map so that you can't get flanked easily, and there aren't as many doorways and what-not for enemies to pop out of and give you a hard time. Make sure you spot them first. You may use a Heart-beat sensor if you wish. It's great because it doesn't even take up a perk slot, only an attachment, and you can see exactly where your enemies are. It even beeps when enemies are nearby so that you won't be at a disadvantage it you have your face buried in it. Also, it forces them to use a perk they may not even want to - Ninja - in order to counter it.
Bonus tip: Use the care package killstreak. It's great because you may get something like a Pavelow, Chopper Gunner or AC-130 without earning it. This means that you can use your luck to your advantage, and changes the nature of the gameplay. Also, make sure you sit in the corner when your killstreaks are out. You don't want to die when your air support is getting kills for you. This way, you can be sure to get a large killstreak. In fact, the majority of your kills mightn't be from shooting guns at all! Imagine that.
So there you have it. Follow these simple tips and you'll be winning and playing like a beast in no time!