-WHEN A SURVOVOR IS BETWEEN YOU AND YOUR TARGET:
The relatively low damage of the pistol means that even if you happen to shoot a teammate, minimal damage will be taken.
If you're slimed and your vision is obscured, look for your teammates' names, they're still visible. Shoot or push when you see motion and you don't see them.

-DANGEROUS WEAPONS:
The following weapons have relatively large areas of effect and thus cause friendly fire havoc. On any difficulty, it is imperative that you know what you are shooting at and have the sense not to stand in front of teammates who are shooting. There are cases where entire teams have been defeated due to taking more damage from friendly fire alone than from hordes of the Infected. On expert, minimizing friendly fire is absolutely necessary to survive as taking even a few rounds from any kind of weapon is enough to take out a huge chunk of your health and possibly incapacitate you. In the worst case, if you are seriously injured, taking even a round or two of friendly fire can kill you outright.

-SHOTGUNS: Do not open fire unless you are in front of your teammates. Generally, do not even try to "help" your teammates by picking off targets that are attacking them. If you must shoot, try to fire at the enemy by having them at the edge of your crosshair so that your shot will just barely hit them. This alone may be just enough to kill the enemy and save your friend. However, it's best to just switch to the pistol, which is far more controllable than the shotgun in terms of spread.

-MOLOTOVS: This is the single most dangerous weapon to your teammates in the game as the fire damage is indiscriminate and has the potential to incapacitate and/or kill a teammate in seconds. It is even possible to incapacitate the entire team at the same time if the molotov is thrown poorly. Always warn your teammates before deployment and aim it far from Survivors. The same goes for the red gas cans scattered throughout the game and are relatively common in the levels. Note that fire damage does not kill immediately, even in the case of the ordinary Infected, much less a Tank. Because of this, laying down a wall of flame that begins six inches from your own face can be worse than useless.

-PIPE BOMBS: This weapon is dangerous for a different reason than the molotov. The blast radius is small enough that it's trivial to avoid but be careful as the explosion can set off propane tanks and gas cans turning that trivial explosion into a seriously damage dealing problem. What is downright impossible is to avoid the horde of Infected a badly thrown pipe bomb will attract without destroying. If needed, throw the pipe bomb a medium to far distance away, so if you don't eliminate all of the Infected, at least they're far enough away to attack in relative safety.

-COMPRESSED GAS CYLINDERS: These come in two varieties, propane tanks (white, fat and stubby, relatively common) and oxygen tanks (green, long and slender, rare). Both of these can be thrown and detonated by gunfire to deal massive damage over a wide radius. That's the problem, in fact -- it's sometimes difficult to throw them far enough to avoid blasting yourself or your teammates. It is not possible, however to detonate these items while they are being held by teammates, should a stray round connect with them. Bear in mind that depending on what you shoot the gas cylinder with, it may not explode immediately. It will usually lose some gas before exploding unless shot with a shotgun or hunting rifle. Additionally, it should also be noted that these objects are highly vulnerable to all sorts of fire damage as well, and will prematurely detonate on their own if they are placed too close to any open flames. Despite the risks associated with these gas cylinders, they are still very useful for set-piece defense situations such as final stands or holding out against incoming hordes.

-SURVIVOR DEATHS:
Close rescue closet doors after you rescue a Survivor. If two people have died close together, the second Survivor will respawn after you close the door. You may need to wait half a minute or so. This is much safer than fighting to another rescue closet.

-MEDICAL ATTENTION:
Whenever possible, try to help fallen comrades up or cover the person helping them up. Remember, this game absolutely loves destroying lone wolves and every time your Survivor count drops you are entering a new world of hurt. When in doubt, just remember: when you're facing endless numbers of Infected, having three live people covering your back instead of two live players and one dead teammate can make all the difference. No one can survive the infection alone.

Help a friend up as soon as they fall and the area is clear. Until the area is clear, cover them. It is impossible to help someone up while Infected are attacking you or your fallen comrade. If there's a Tank and he is far away, go ahead and rescue.

It is imperative to understand that if you are helping a Survivor up who is not being attacked and another Survivor gets pounced by a Hunter, help the pounced Survivor first, especially on higher difficulties. This also applies if a Smoker constricts a teammate.

Once you sustain damage, you can regain health with:

-FIRST AID KIT:
Stabilizes your health and puts you into the green (80 or better hitpoints).
Pain pills: Adds 50 temporary health points to your current health. In addition, having at least 50 health will allow a survivor to run at full speed.

-ADRENALINE/ADRENALINE INJECTOR:
Adds 25 temporary health points, which decay at a slower rate, to your current health. In addition, fatigue does not act against you when shoving, you are not slowed down when hit, and you move much faster.
Defibrillator: Commit suicide. Upon being brought back to life, you will have 50 health points that do not decay. Another perk to this method is that you can pick up the items you had when you died.
Dying then getting "rescued", you get 50 hitpoints when your closet is opened. You with also be assigned a tier 1 weapon, and a single pistol.
Although it is not entirely helpful, when on temporary health of less than 30, incapacitate yourself by hanging over a ledge. When you are helped up, your health will go back to a temporary 30.

-BEING HELPED UP:
Restores you to 30 hitpoints that decrease down to 1 hitpoint over time.
Only works up to 2 times unless you use a first aid kit, which resets the count. You die on the third time. It's easy to tell because your screen goes black and white and your heartbeat gets loud. Make sure to tell teammates, as it is less obvious for them, with the only cue being character dialouge.
Death is not permanent. How to return:

Your teammates will need to advance a certain distance beyond your body into the level.
You will need to wait 1 minute
Your teammates then need to open the closet you will spawn in to complete the "rescue" of a "new" Survivor. It's always best to close said closet after rescue, as on some levels the same closet covers a wider than average length of the game. However, in Versus, Survivors are not allowed to return and will not respawn.
In Left 4 Dead 2 you can be revived with a defibrillator and return to the game in the same spot where you were killed.
Sometimes after completing a level in Left 4 Dead.

*...USE THIS KNOWLEDGE TO GAIN LEVERAGE OVER YOUR OPPONENT AND ACHIEVE OVERALL VICTORY...*