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You can unlock all sorts of perks in multiplayer as you level up and there are also rewards for kill streaks depending on how many bodies you stack up. There are even options which open up when you get killed several times in a row which help get you back in the game.
Think about what to use. Also, don't feel like you have to unleash kill streak rewards immediately. It is better to wait until the right moment to call in an air strike or even get a care package. The Sentry Gun is a kill streak reward but can also be unlocked as you level up. It is well worth getting.
Camping sucks so I don’t recommend sticking to one spot for too long, but be aware that many people do exactly that. It is good to pick a vantage spot for a couple of kills but move on after that or you run the risk of being uncovered. You should avoid running around too much and always keep an eye on corners and windows. Remember to think about where you were shot from. When you respawn head for cover and if you know where your killer was go for revenge.