Communicate-
Communication with your team is crucial in any multiplayer game. To find out if anybody on your team gives a damn, speak clearly and speak often about your intentions and your plans. If there is anyone else on your team willing to cooperate they will make themselves known quickly. It is important that you don't come off as a jackass too, people like that tend to get ignored.
If you're a survivor, tell your team to stick together. If you venture too far away from eachother you will be easy pickins. It's also important that you announce your position clearly whenever you get seperated from the group.
If you're the infected and it's not been done already, attempt to coordinate attacks. Even if you only have one person working with you, you will be far more effective. Be constantly vocal about your plans until someone bites.
Be sure to quickly let your team know if you get caught as a survivor. Once a charger grabs you, for example, you'll be dead without your team's help.
Strategize before the match-
Come up with a plan before you begin. Randomly attacking the survivors with one beast at a time will usually get you nowhere. Think about angles and multitier attacks on the survivors before you start.
If you're a survivor, devise plans of defense based on the map. Think thoroughly about the best places to set up defenses or perimeters to cover all sides. Ideally you'll want to funnel the bad guys into a choke point, while maintaining escape routes when possible. If you think about these things before you start you'll be able to place your focus on the killing.
Familiarize youself with the infected
The hunter, smoker, jockey, and spitter are weaker beasts and thus should be used in conjunction with more formidable foes. Use the tough guys to grab the survivors, then the weaker infected to assist in the destruction.
Use Tanks wisely
Tanks are extremely useful. Don't mindlessly charge into the survivors without a cause. Use the Tank to launch boulders and cars at the survivors to not only soften them up, but also as a distraction to allow other monsters to move in for the kill.
Anticipation
It's always important to think ahead about what your enemy is going to do. This is true for most multiplayer games and sports. Think about it like chess where thinking one move ahead restricts you from setting up the check mate. Try to think several moves ahead to crush the enemies dreams.
The spawns are yours
Infected can't spawn near survivors, so try to maintain positions that force the infected to spawn in less desirable areas.
I hope this helps some of the newer players to the franchise. I am no pro at l4d2, but these strategies are applied to all multiplayer shooter, for the most part. Work on these and you'll be winning in no time.
