Modern Warfare 2 has a much harder ranking system than previous Call of Duty games. Average gamers take 1-2 days play time to make it through one prestige.

I am going to start with the basic things you should have.

-Try to play Demoliton, Domination, and Ground War. These are all good because of lots of kills/killstreaks and demolition has INSANE match bonuses.
-I have notice as you reach higher levels match bonuses increase too, by the 60s demolition can easily give you 4000+ xp per round off match bonus alone.

The guide gets rather lengthy here so I will number the sections.

1. KILLSTREAKS
2. CLASSES
3. TECHNIQUES
4. CHALLENGES

1. KILLSTREAKS:
Try not to use nukes EVER, they encourage camping too much when they really don't get you many kills or points.

Good setups include:
-Predator, Harrier, Airdrop/Chopper Gunner/AC130/Pavelow
This is my basic setup. This is good if you are able to make it to the Preadator Missile fast because you will easily get your Harrier afterwards.
Emergency Airdrops and Pavelows don't count toward killstreaks, I've heard of other people having luck with pavelows, but I don't prefer using it unless its a top killstreak. Chopper Gunner is better than AC130 but the AC130 challenges are worth it.
-Care Package, Predator, Harrier
For when you can't get very large killstreaks easily and carepackages tend to get you enough high killstreaks to even out the low ones.
-Predator, Precision airstrike, Emergency Airdrop
Gives you more instant gratification when going for airdrops.

2. CLASSES:
For the first few levels try to use Marathon+Lightweight setups until Hardline is unlocked.

My favorite classes include:

UMP .45 silenced
.44 Mag Akimbo
Marathon
Lightweight
Commando
Semtex
This class is one of my absolute favorites, you can easily flank around a team before the first blood and get semtex multikills. The silencer makes a MASSIVE difference because people rely on radar to unbelievable levels and when theres no red dot, they wont care. .44 mags rival shotguns, have great hipfire, 2 shot kill w/o stopping power, and you can switch to pistols in seconds.

Any full-auto Assault Rifle Silenced or Red Dot
Striker, Spas, or 1887s
Scavenger
Hardline
Ninja
My basic assault class. Absolutly all assault rifles don't require Stopping Power (except Tar-21 I will explain later) Basically if you aren't getting shot, it wont matter how fast you can kill someone because they will die either way.

Sniper:
Intervention/.50 cal Thermal or WA2000 silenced
Striker, Spas, or 1887s
Scavenger
Stopping Power
Steady Aim Pro
Claymore
You can hang out far away with you back covered and an unlimited supply of claymores. Try using WA2000 silenced, when you get used to shooting to upper chest you will see the difference you can get with the silencer.

Tar-21
Shotty
Scavenger
Stopping Power
Ninja
I find that the Tar-21 is the only assault rifle you need with Stopping Power with because it kills in 2 shots with stopping power and without it that third shot is always hard because of the high recoil. This class is great for Free-For-All.

Try to not get used to using Stopping Power because its just something that will annoy you when you take it away and when you think of it, with Hardline, Chopper Gunner is 10 kills, not a big difference. But when you think of it, 4 kills to Predator which gives you Harrier, which gives you Chopper Gunner and 4-5 kills is most peoples limit before they die when flanking.

3. TECHNIQUES
In all gametypes I play, I either camp or flank. If you find yourself in a game where you are constantly dying and your team is being destroyed. LEAVE and find a better one your just bringing your hours up.
When flanking try to find all the openings where you can sneak behind and pick everyone off racking up your killstreaks. When having a small 1-on-1 with an enemy play and you run out of ammo switch to your shotgun or handgun. If you get into these situations a lot you should use handguns more because you can switch to them faster.

Camping on the other hand. Try to find decent spots in the map where you can stay protected and see over the battlefield and pick people off. When you find your spot is getting slow, move up to where the action is at and move back when your team is getting pushed back. You don't want to get stuck in a situation where you can't go anywhere and are 1 kill away from a Chopper Gunner. Playing aggressively while camping is still very possible because you are helping clear the way for your team for them to move up.

With the strategies, granted you are a decent player, you will score 2-1 kill death ratios in each game.

4. CHALLENGES
THE ABSOLUTE MOST IMPORTANT PART OF RANKING UP! Doing challenges will have your rank flying.
For guns, dont bother with side arm challenges too much because they aren't worth much points, except for shotguns. For primary weapons, use your favorite weapons first and go for the 300 kills to FMJ and get over 17000 xp for those kills. Don't waste time with mastery because you will find yourself wasting time trying to get FMJ kills that you can't get. If you find yourself good at these then do it.
For perks, the easiest perks to finish are:
Commando
Stopping Power
Hardline (just use UAV, Care Pack, Preadator)
Scavenger
Sleight of Hand
Steady Aim

Some of the hardest are Marathon, Lightweight, and Sitrep, don't really try to hard for these. Some perks like Ninja you will need to get the pro version to do very good, get them.

Killstreak Challenges are easy xp for general gameplay, 25 Chopper Gunners is an easy 10000k.

Try getting the easy general gameplay challenges too, Crab Meat is 7500 xp for something you can do with a pavelow.

Personally I'm a hacked 10th prestige and plan on making another account to see how fast I can rank up doing this. I would go in depth more but there's not much more I can say. Try to reply and get this thing known for other people.