Battlefield 3 is not a game well-suited to FPS players who only care about their personal kill/death ratio. In this way, it's a very different type of game than its main competitor, Call of Duty: Modern Warfare 3. Whereas players with the quickest trigger finger will often emerge on top of the heap in Modern Warfare 3, a match in Battlefield 3 completely breaks down without good team cohesion.
Often "good cohesion" simply means that players are filling roles that need to be filled, and performing meaningful actions to put their team ahead (instead of just shooting all the dudes in the sight). In some cases the difference between a win and a loss can depend on a single player who fills the right role at the right time. I've put together a list of ten rules that, if followed, almost guarantee that you'll start to earn more points, more wins, and more friends. In short, these are the sorts of things that will make your teammates very happy.
Shut Up and Get in a Squad
This is the golden rule of competitive Battlefield 3 play. Without exception, you and your team will always perform at a higher level whenever you're in a squad. Every time you spawn on a buddy while he's taking an objective, he'll be so happy that he'll want to teabag an M-COM station.
Plus, whenever you're in a squad, others will have the ability to spawn on you and help out with whatever it is you're doing. And remember, more is better; if you can join a squad that already has three other soldiers, do it.
Don't Fly That Jet
The next time you're spawning and see that there's an F/A 18 Super Hornet ripe for piloting, step back and reconsider for a second. Ask yourself a few questions:
Sure, jets can be useful for your team, but only in the hands of a player who knows what they're doing. Unless you believe that you can successfully defend your team's helicopter or take out an enemy jet that's bombarding your ground troops, everyone will be a lot better off if you take a ride in a tank instead. And anyway, you're likely to get a lot more points if you keep your feet glued to the ground.
Tanks Need Friends
This one is pretty simple. A lone player in a tank is going to get taken out pretty quickly. A tank with a least one other engineer (or another tank) providing backup can completely dominate an entire team. Friends don't let friends tank alone.
Be the Backup Guy
Throw down med packs and ammo packs if you've got 'em. This is probably the most common piece of advice given to players who want to become better teammates in Battlefield 3. If you see a dead homie, slap him with your paddles to give him the gift of life. If you're playing with an LMG, don't be afraid to spray wildly at those guys hiding behind cover. You'll likely keep them suppressed, allowing your teammates to take them out. It's not always about being the hero. However…
Be a Hero - Hit the Big Targets
Sometimes your team needs a guy who can handle the heavy lifting. It's important to provide support, but if there's a deadly enemy tank going unchallenged, you need to do something about it. The worst thing you could do would be to simply hope that "somebody else gets it." Be a hero; switch to Engineer.
Don't Blind Your Bro
Turn off your flashlight/tactical light when you're looking at your teammates. Please. You're making them want to team-kill you.
Reconnoiter Like a Real Recon
Notice how this class is called "recon" and not "sniper?" Yeah. Most people don't.
If you're ever on a team that's getting completely annihilated, just do a quick look around. You'll probably find that half of your teammates are camped out on rocks near the back of the map, too busy missing targets with their sniper rifles to do anything useful. These people are the absolute worst type of teammates. They contribute almost nothing to the effort of the team as a whole, and usually wind up with a kill/death ratio no better than 1.00.