First things first
You NEED a team with microphones.
Communication is VITAL

Second
Call out any and all findings.
Remember what I said about communication? This is where that comes into play.
When you find weapons,ammo,equipment,or health kits, call it out to your team.
A well supplied team is a successful team.

Third
Stay with your team! If you get constricted or pounced, and your team isn't around, you're screwed.
Also, teammates are great at helping you fight off a horde or tank.

Fourth
Use that left trigger!
If you see a teammate pounced or constricted, don't shoot the hunter/smoker, shove them and THEN shoot.
It takes a few shots to kill a special infected, by the time they're dead your teammate is down anywhere from 5-20 health. If you see a teammate surrounded by the undead, shove them away and then shoot, you may think you're helping by unloading into the crowd, but you normally do more damage to your team with friendly fire than you're saving them.
REMEMBER, Shove then Shoot!

Fifth
Don't be a dick.
If you see a teammate pinned, don't mock them while they're down, as you may be relying on them to save your ass from becoming undead cuisine. If you see a team member with low health, and yours is in the green, heal them or give them pills. If you keep them in good health, they can watch your back.

Sixth
Use power shots wisely.(There is a glitch that is unpatched so far, it allows you to fire an entire clip worth of ammunition in less than a second. It only works on the Uzi and Assault rifle. To do this glitch, Hold left trigger with your pistol(s) equipped, press Y while holding left trigger, Hold right trigger for a few seconds, then release left trigger.) A power shot can completely destroy a witch if aimed at the head, but be careful, if you miss you're stuck with a reload and a pissed off witch. Power shots chew up a lot of ammo, so don't use them on common infected. An entire team of simultaneous power shots can kill a tank instantly. You should only use power shots on tanks and witches, as they use an entire clip and are insanely overpowered against common infected and special infected.

Seventh
Use equipment wisely!
If you're in the middle of a horde attack, a well placed molotov can turn a "I just shit my pants" moment into a "I just shit my pants laughing" moment. If you're surrounded by a large amount (15+) of zombies, sometimes the best thing to do is throw a molotov at your feet. Don't forget about pipe bombs either, they're great for making a break for it, and if you're low on health, use those pills!

Eighth
Don't go idle if you can help it.
There's no communication with an idle player, and the A.I. does the stupidest things.
So unless there's something important, don't go idle and let your team down.

Also, I turn 15 today, Happy birthday to me.