Knowledge of spawns and decent reflexes/reaction times are both vital to getting MOABs. Also (while it's not vital), having either headphones, a Surround Sound set-up or Surround Headphones really help. I was watching a gameplay of my friend trying for a MOAB; at least 3 times I heard someone moving around when he ended up walking straight past them (e.g. he was running under the covered bridge at the bottom area in Carbon, and I heard an enemy walking on the bridge itself).
Caution:
If you think there's 10 guys camping behind one building then don't go that way. You'll be lucky to kill one or two of them before you get skull ****ed with virtual bullets. If you think the enemy has the upper hand for one route, find another. ADS around corners, wait for teammates to go around the corner first. This may be a pretty selfish tactic, but you'll be able to see the general direction of the bullets and where they came from. I use this tactic so much it's not funny.
I nag my (aforementioned) friend to be more cautious when playing; don't rush. I cannot stress this to him enough. He goes along saying "If FearCrads can get a rush MOAB, why can't I?"
I reply with, "You've gotta crawl before you can walk."
My proof: His sensitivity is around 2-4, and he's never gotten a MOAB in his live (apart from when we did a private match to see what it looked like
)
Back to the sensitivity. If you have a low sens like 2-4, practise raising it by 1 or 2 every week or so and settle on something around 7 or 8. I consider myself a decent COD player (1.8 KDR, almost entirely Specialist) and my sens is around 7. Because you have a higher sensitivity, you can react quicker to situations where a lower sensitivity would get you killed. An example would be: You're running down a corridor and thanks to the atrocious spawn system by IW, an enemy spawns right behind you. He starts shooting when you can easily turn around, drop to the floor and shoot him square in the chest for an easy kill. I've done this so many times before as well, so it works.
If you're not using the Tactical controller layout, do so. In high-risk CQC to mid-range firefights, dropshotting may be fairly cheap but it's really helpful. Practice pressing down on the right thumbstick when you start shooting. Your enemy has to aim down against the recoil, so most of the time, people overcompensate. By the time they've corrected their aim, they're already dead thanks to you.
Now, for a bit of psychological tips:
Never aim for a MOAB right off the bat. If you've only gotten a few MOABs before, or haven't even got on yet.. don't jump in with this type of mindset: "OMFG MOAB TIME BITCHES!!!! TIME TO GET RAPED BY MY SKILLFUL FMGs"
Two things:
Don't expect to get a MOAB right away. You set the bar too high and you'll end up messing up your pants because you're shaking so much.
You're a noob for using the FMGs. They take no skill to use and they're overpowered. Please snap your disc of MW3 in half.
If you do happen to get on a decent killstreak, don't count and don't check the scoreboard. You'll end up thinking about how much left you have and not thinking about whether there's an enemy behind you. Also, don't think, "Only one more now. *kill*, OK, only one more this time. *kill*, I mean this time!"
Like I said before, don't expect anything.
Don't blame your death on the enemy. If he was camping in a corner using Assassin Pro, then you should have checked your corners. If he sniped you from across the map, you should have known about most sniper lines of sight and gone another way. Someone knifed you from behind? Spawns flip all the time in MW3, if you're going for a MOAB you should know that by now. Always check your ass to make sure someone's not gonna stick their blade in it. And then stick their virtual testes in your virtual face that is virtually dead.
If the enemy has Assassin, Steady Aim and FMGs; then by all means blame him because he's being a nooby asshole.
If I think of anything else to add then I'll probably add it in here. And if you don't agree with my hatefulness towards Assassin Pro and FMGs, so be it. I just don't like them. Just don't call me a hacker when I obliterate you with my .44 Mag and Throwing Knife
I feel sorry for them
that I can do on my mobile phone anywhere in the country, and each post taking no longer than a minute, which is a WHOPPING 6 minutes per day!!! 
