I've got to point out that while we're throwing out information to help you guys, our methods just might not be effective for you. We've all developed our play style through trial and error I'm sure, and kept what worked and ditched what didn't. Trying to stay on the perimeter and keeping your back to the boundaries is definitely a good way to cover your tail. Of all the accolades in the game, 6th Sense is the one I have to have the most of, which is for no deaths from behind in a match. It's either that or Undercover which is for being close to the enemy most. Using the map terrain and buildings to your advantage is a big key in doing well. Minimizing your time in open areas will keep you from getting shot at from 4 or 5 different angles. Be fluid in your movement, be able to flow and adapt. You don't want to get dead set on going to a particular spot a particular way only to be surprised by someone. Too many times I have found myself focused on flushing out one person only to miss the guy running at me from the side.
As with classes and weapon setup, I swear by the silencer. There is no fear of being spotted on the map when you fire, so you don't get people rushing at you when they see your dot near them. Also an effective way to take out multiple enemies in a short period of time before they notice their buddies drop beside them. They'll have to guess which direction to face. Plus it helps with recoil. ACR and TAR are my go-to guys. The ACR is a bit more accurate at longer ranges, but the TAR has higher damage. The perks I can't go without are Cold-Blooded Pro and Sit Rep Pro. You can use any 1st perk that suits you, I happen to choose Scavenger because I'm so awesome I'd run out of bullets without it. I run an OMA class as well, but that's just for adaptability as I can drop some claymores then switch to whatever other class I need. Ninja Pro would probably be the better 3rd perk for most because getting to Sit Rep Pro takes forever. I don't care for Stopping Power, I don't use it, I will gladly sacrifice that 2 or 3 bullet advantage to be able to roam the map without having to worry about UAVs and air support. I might get 3 or 4 kills instead of deaths with SP, but I'll also probably get killed 3 or 4 times by killstreaks.
When I first started, MW2 was the first FPS I owned on a console that had usable multiplayer (I had one of the CoDs on PS2, but you needed the modem thing for multiplayer). I had played MW split screen and I sucked and this was no exception. I was probably about a 0.75 - 0.9 player throughout the first 70 levels, although I was closer to .6 because I started off so bad. I worked it up over 1.00 through the first prestige and I've gotten it up to 1.5 total now, and I'm closer to a 2.00 - 2.5 on a regular basis, and on good days well over 4.00.
And for the record a6 if you must know, you may call me Dave. Just don't ever go all HAL 9000 on me or I'll beat you. Also, holy cow I typed a lot.
TL;DR- Finding what works takes time, limiting your blip on the radar is a big help, be adaptable to what's going on in the match.