What To Do and What Not To Do(For Noobs)
Don't run down an open lane, find at least a bit of cover when running. Long open alleys allow a sniper to shoot you in the face easily.
Don't try to just sit still and hide behind a box, often times what you think is cover is half your body sticking out in the open and an easy kill for anyone fast or a sniper. The only acceptable time to really camp anyways is during Search and Destroy or if you are sniping.
If you use push-to-talk for external things like Ventrilo, having your talk button where you have to remove your hand from either your mouse or remove ability to walk/aim and talk is always a bad idea (sorry Maria

). I suggest removing your grenade throw from your middle mouse click key binding and make your middle mouse click your push-to-talk, it's easier and you can still aim and walk while talking.
Do not rely on Noobtubes (rockets launchers, grenade attachments, etc.) / G18 Akimbo, they are great for when you first get the game and you feel like you fail cause you can't aim and get a kill, so why the hell wouldn't you use a gun that allows you to shoot and kill everything within a 15 meter radius?
Well for one, you will never learn to actually aim that way, practice shooting, play through the entire campaign. Before I even played multi-player I beat the entire campaign on Veteran (only took 6 hours or so if I remember correctly and I died quite a bit). If you are new to FPS then I suggest at least beat the game on the normal mode. Not only will you actually learn to aim somewhat, but you aren't doing it against someone who is 1000x better than you are, you won't be going 0 and 15 anymore.
Create your custom classes to reflect the game types you like the most, always have a class that has your favorite assault, one with your favorite support type such as a stinger because the regular launcher sucks, I have one that is specifically built for capture the flag as well as a tanking class that can be used for headquarters (shield and cold blooded). Since I am a sniper I have two of those classes, one with an acog and one with the normal scope with a silencer. The acog allows quick aiming while not narrowing your site down to a 1 meter radius. It basically turns it into a single shot assault rifle that can easily kill in two or even one hit. Some people don't like using the silencer because they say it reduces the damage it causes, well if you don't suck at aiming then you can shoot their heads, or put two bullets in them before they can respond. It won't reveal your position as well if you're hiding in a great spot.
I don't really use thermal scopes, not because they're hard to get (because they aren't) but because in certain maps they do not help at all and can actually hinder your ability to find people (I'm too lazy to change my class configurations between matches most of the time). If you really want to be a good sniper just be able to notice tiny movements in the grass or start to learn where people are, people are predictable and will do the same thing over and over again.
I still choose revolver over all the other pistols when tactical knifing, I don't know how well the Deagle fares but if you can at least aim with a gun, two shots with a revolver will take them out quickly and you can go into cover. If you happen to appear infront of an enemy while tactical knifing you should strafe back and forth to the sides depending on how far you are from them, noobs will hipfire and waste all their bullets hitting the ground or the clouds allowing you to get close for the kill. Also try to learn the distance of your tactical knife, commando pro + tactical knife gives you about 6 feet of jump with a knife kill. I don't recommend knife only in hardcore game modes, not only does it take only 1 or 2 shots to kill you (hipfire WILL rape your face) and you are usually playing against people who are more skilled / experienced; such as veterans from COD4.
DO NOT CALL EVERYONE WHO IS BETTER THAN YOU A HACKER.
We'll take it as a compliment but it is quite annoying; it's very possible for someone to basically rape your entire team without dying. Maple and I both have 3 tactical nukes under our belts which we earned legitimately. It is quite possible, just doesn't happen that often.
Lag and Latency Displays
You can figure out the host by using this trick as well as being able to tell who is lagging hard. Normally your latency is displayed using four green bars, total they're 400 ms of ping, or 100 each. What this means is that a player with even three green bars could have 100-200 ms ping and still have three bars, this is actually TERRIBLE latency when playing FPS games, especially since the tickrate is set at 30 default.
Finding your Multi-player Configuration File
If you navigate to your Steam folder in "Program Files" or "Program Files (x86)" you can find another folder inside called steamapps, inside that is another called common, and inside that is where the modern warfare 2 configuration files are stored as well as the game files. In the call of duty modern warfare 2 folder there should be a folder called players. In this folder resides a file called
config_mp.cfg open this file with either wordpad or notepad, I recommend wordpad or Microsoft office.
The path will look something like:
C:\Program Files (x86)\Steam\steamapps\common\call of duty modern warfare 2\players
It may differ depending on where you installed the main folder of Steam to.
After opening up the document scroll down or do a search/find for the lines that say:
seta cg_ScoresPing_MaxBars "4" (change the 4 to 10)
seta cg_ScoresPing_Interval "100" (change the 100 to 20)
You can edit these values to whatever you like actually, but I think the maximum amount of bars are 10. If they aren't displaying correctly try lowering the bars to fit your resolution. I run at 1680x1050.
What I've done is change the max amount of bars displayed to 10, this allows for a more precise reading of pings when combined with the ping interval from 100 to 20 ms. So full bars = I have <20ms ping, and for each bar that is gone I have 20+ more ms. (Example: 3 bars gone so I have <80ms ping) allowing for a max of up to 200 ms being displayed if only one bar is showing.
The host will always have full green bars (10) so it will be easy to tell who is host now and who is actually lagging. People will still say they aren't lagging but that's because they're idiots.