Ok, there are a few tuts on here on how to use an online auction site to get double xp, but I am going to make this one quick and easy.
You first are going to want to go here (Yes its a referral link, but if you use it you have a better chance at getting a code.) [ Register or Signin to view external links. ]
tia.com/signup/915306 (you need to remove the space between the "s" and the "t")
Once you sign up, search "MW3" and a whole bunch of auctions will come up for double xp.
Next use your 600 FREE credits that you got from using my link, and bid away! Hope this helps you guys out .
UPDATE: If you need more credits, list a fake auction, or just create an auction of something you want to get rid of