Call of duty modern warfare 2 how to level up quick

If you are playing online and you have a friend online you can do a thing called 'Boosting' where you can play a 3rd Person Cage Match and let the other player join your session.
Once this is done the first player gets 19 headshots and the second gets 20 you can switch turns and do different challenges to earn extra points and you should get past levels quickly.