Instructions
Don't waste bullets. The fastest way to level up in the game is to go on multi-kill streaks, where every bullet connects to a target and you wipe out all targets without getting hit yourself. Do not fire your weapon aimlessly. Streaks rely on your ability to be precise and methodical with your kills.
Don't be afraid to snipe. If you have a little patience, hiding out in a corner with a sniper rifle can boost your kill streak score and will also help you earn special weapons-based XP. Just make sure to check your six every five seconds so you don't get offed yourself.
Kill enemies in groups. Double- and triple-kill bonuses are huge XP boosts, and you can use grenades or rocket launchers (if you have the bomb squad perk unlocked) to kill many enemies at once. A group kill also can kick-start a kill streak, which will give you even more XP bonuses.
Take them with you if you are going to die. If you find yourself surrounded by enemies and you're going down, throw a grenade right before you respawn so you can get XP for a kill even as your character is dying.
Play special modes. Team Deathmatch is always the most popular online mode, but if you are looking to level up as fast as possible, try playing the Domination and Headquarters modes for goal-based XP bonuses. Do not play the Search and Destroy or Hardcore modes because they do not offer substantial bonuses for accomplishing goals, and their formats aren't conducive to getting large kill-streak opportunities.