How can you level up fast in call of duty modern warfare 2

I would suggest playing Sabotage or Demolition. Then just sit back and pick off the people running to the bomb site. Players are more concentrated on the objective than they are killing you. You can get a lot more kills this way and level up faster.

Or you can get a friend and both have a riot shield class and a machine gun class with a big clip and shoot at each others riot shield,you gain like +100 every thirty seconds.