Find a favorite weapon. Not everyone is good with the same gear. Figure out what works best for you, and then use it even if people try to make you feel bad about your success.
Cover fire is effective. When your team is approaching key enemy territory, come in with guns blazing to force your enemies into either giving up important ground or takingsome bullets if they emerge from cover. Either way, you've given yourself more of an advantage on a tough approach.
Even when there's action everywhere, don't forget about objectives. If you stick to those objectives, you'll force your enemies to respond instead of allowing them to plan unknown chaos, and that keeps the battle on your terms.
It's all about headshots. Even though the chest is a nice big target, get used to aiming for the head to score vital instant kills.
If you can afford grenades, always buy and use them as they are a great way to force enemies to retreat or out of cover. Any of those outcomes can easily work in your favor.
Keep your eyes and ears open. If your sound system allows, you can hear your enemies reloading or fleeing the scene. That's useful intel that lets you know when to rush and when to back down.
Monitor your radar for important clues as to your enemy's whereabouts and likely movements. Since you can't actually see everything that's going on on the map, it is a resource you don't want to neglect.
Loot corpses, especially teammate who have good gear. Stick close and grab stuff when he gets shot down to save you from having to invest in first-rate equipment for yourself, if money is tight.
Learn from better players by watching them on the free camera, or even jump into the first-person perspective of one of your more capable teammates. You may learn a new strategy or little tactic that could improve yourself as a player.