Left 4 Dead

Survivors

Don't shoot your damn teammates.

Try to crouch as much as possible while in hallways to prevent FF.

Do NOT run wildly in front of teammates that are shooting.

If a Smoker or a Hunter has a hold of one of your teammates, you CAN shoot your teammate to break them off... just stop as soon as their loose (it only takes one shotgun shell up close).

Try to set tanks on fire. They die MUCH faster that way.

Never, ever, ever find yourself alone. If you're bitching that your teammates are "leaving you behind" then move your fat ass faster.

Corners are your friend. If you get boomed, or there's a horde coming in, get yourself and your teammates into a corner. Also, while in said corner, right-click like crazy. Your "punch" is really useful.

Lastly, listen to the veteran players in multiplayer. They know what they're doing (usually).

Don't get too far ahead or behind the rest of your group; this may sound like common sense, but it's also a common mistake. Two of the special infected (including the hunter, the most common one) can disable you, and you need someone else to save you.

Conversely, if someone else runs ahead or lags behind, it's best if the group lets them catch up by either slowing down their progress, finding somewhere safe to hold out for a minute or going back to retrieve them. Ideally, you always want to be together as a group of 4.

Learn where the possible spawn points for level 2 weapons are and don't miss them.

Witches are much less intimidating when you have an autoshotgun; the correct method for 1-hit-killing a witch is to crouch and move towards her, wait until she starts to get up (her knees have to leave the ground, but you have until she stands hunched over for a moment before attacking) and fire one shotgun round at point blank range. This works for either the pump shotgun or autoshotgun. Hitting anywhere on her body works so go for the stomach/chest. Witches don't take more damage from headshots.

In Versus, always, always set the tank on fire. A burning tank will die in 30-40 seconds regardless of any other damage done to it. In Campaign, setting a tank on fire causes it to speed up, so unless you're a long way away from it, it will outrun you before it burns to death.

Don't miss pipebombs and molotovs, they're amazingly useful. Molotovs are best used to block choke-points, especially if there is a horde coming or a special infected chasing you; it'll have to wait until the fire's gone. Pipebombs are useful when you get biled by a boomer; unless the infected are already attacking you, a pipebomb takes priority over the bile.

If everyone still has medkits and you find some, the person with the lowest health should heal up and grab the medkit.

Never underestimate the usefulness of pills. The medkit tip also applies to pills. If your temmate has less health than you, give them your pills. They don't have to use them until they're needed, but you don't want to come under attack then have to find and give the pills when the teammate has 5hp left and you're all blind. If you take some damage, they can always give them back to you.

If you get to the ending saferoom and others are lagging behind, close the door. It's easy to reopen from the outside, and all doors in the game open both ways.

If you know you're within 30-40 seconds of the safehouse and have near full health, run like hell, especially if you know that all four are either dead or spawned or both. People generally get too fixated on actually killing the special infected TEMPORARYDASHCHARACTER if they're alive and behind you, they're not in front of you killing you.

Learn popular infected spawn points in versus, then as survivor, make sure to cover those spots. Infected can only spawn in places where they're out of sight. It's possibly to completely shut infected out from spawning in certain key areas, making your run easier.

If a teammate gets hit by a boomer, the ensuing zombie rush will ignore all teammates who did not, meaning you can just stand right in front of them shooting wildly. You'll take most of them out without taking a hit. Conversely, if YOU'RE the one who got hit by the boomer, your best option is to find a wall, crouch against it, and switch between melee and your gun to keep the zombies at bay.

Always stay in close proximity (and preferably visual range) of your teammates. Separating will almost always get you killed or incapacitated. Remember that online, there are people controlling the special zombies, and they know how to plan ambushes and set traps.

Watch for teammates behind you that have been incapacitated by a hunter or smoker. More often than not, there will be a second one waiting for you to rescue your teammate.

Automatic weapons (uzis, assault rifles) are the best guns for new players. Once your aim improves, you may want to switch to the shotgun/hunting rifle.

If you're not wounded or really low on ammo, don't explore. All the various rooms and passageways you come across MAY contain medicine/ammo, but more often than not they'll just contain more enemies.

Pipebombs are extremely handy when hordes are on their way (either because you summoned them or someone got hit by a boomer). As soon as you hear the music or the noise of the horde, toss the pipebomb in their general direction.

Along those same lines, if a teammate got hit by a boomer, the ensuing zombie rush will ignore all teammates who did not, meaning you can just stand right in front of them shooting wildly. You'll take most of them out without taking a hit. Conversely, if YOU'RE the one who got hit by the boomer, your best option is to find a wall, crouch against it, and switch between melee and your gun to keep the zombies at bay.

Fire is one of your best bets for taking out a tank. Once a tank is hit with fire (from a molotov, gas can, propane tank, oxygen cylinder, etc.), they only have 30-40 seconds to live, regardless of how much or little damage they take from you and your teammates in that time. If you can't hit the tank with fire, shoot it until it targets you, then run and let it chase after you. If you aren't too wounded, you'll be faster than the tank and your teammates can pick it off from a distance.

When you hear a boomer (they make gurgling noises) immediately attempt to find it and kill it from a distance. If the boomer gets close, use your melee attack to knock it back. Never shoot a boomer in close quarters if you can avoid it.

Always keep an eye on your teammates (provided they aren't total deadbeats). More often than not, they'll be working together to take up strategic positions during chokepoints and horde rushes. If all four of you are crammed into a corner somewhere, which will happen often, watch your teammates. If they stay standing upright, crouch down in front of them. If they're crouched, get behind them and stand up.

Infected

Always pay attention to the colour-status of the survivors. Green survivors are healthy, yellow/orange are hurt, and red are near death. Purple survivors have been hit by a boomer. Coordinate your attacks accordingly.

Pressing E will respawn you to a location closer to the survivors if you fall too far behind (or if they surge forward, like in the hospital elevator).

If you're playing as a hunter, your jump will be your most effective form of travel, but you need to be crouched to use it. Also, while crouched you emit a low growling noise that survivors can hear, but while upright you're entirely silent.

If you get to be the tank, focus on the upright survivors. Once you knock someone down, you do far less damage to them, so move on to one of the still-standing survivors.

Smokers work best from heights and from behind corners. You can drag survivors a long way, so the harder it is for their teammates to target you, the better. So try to get on top of buildings or above holes in the floor/ceiling. One of your best tactics is to wait until the survivors hole up somewhere, then drag one of them out, forcing at least one other survivor to rush out to provide cover fire for their teammate.

Hunters should watch for survivors that are wounded or lagging behind. The farther the survivor is from their teammates, the better.

Boomers can vomit on survivors, and this ability takes a long time to recharge so use it carefully. A good idea is to try to get ABOVE the survivors, drop down in the middle of the pack and spit on them. Even if you're waiting for the spit to recharge, you can still coat the survivors with spit if they shoot you in close proximity, so always try to get as close as possible to them.

Remember that when survivors have been hit by a boomer, they cannot see and won't be alerted if another survivor is incapacitated. This is the perfect time, as a hunter or smoker, to pick off any survivors that HAVEN'T been hit by the boomer, as their teammates won't even notice for another 10 seconds or so.

Ideally, as a team you want to take down as many survivors as possible at the same time. Example: If a survivor is straggling, use a smoker to drag him even further back. He is then perfect bait for your trap. Boomers can either vomit on him to get a horde in the area and deal more damage, or wait in the area and get the survivor/s who come to the rescue, prolonging the attack. If one survivor comes to save the first, a hunter can incapacitate him quickly, meaning another survivor has to come and get the two of them. By this point, one survivor is usually dead.

Always pay attention to the colour-status of the survivors. Green survivors are healthy, yellow/orange are hurt, and red are near death. Purple survivors have been hit by a boomer. Coordinate your attacks accordingly.

Boomer: Your death explosion doesn't have a very long range, and any active stream of vomit vanishes when you die. Either sneak up on the survivors or get the drop on them. You get credit for damage that normal infected inflict on survivors that are under the effect of your vomit/explosion.

Smoker: Whenever possible, smoke from a different level than the survivors; either above or below. When you have a chance to target survivors that have been hit by a boomer, go for the one(s) that didn't get hit. If you go for the blind one(s), the others can free them from your tongue easily. Also, don't forget you are quite audible.

Hunter: The further you travel in the air, the more damage your pounce does. You do significantly more pounce damage when on fire. Your claws do more damage than the smoker or boomer's. Also, you can somewhat blend into a mob of normal infected when not crawling.

Tank: Don't get set on fire. Don't try to squeeze through small openings. If you can hit survivors with a physics object (glows red while a tank) it will instantly incapacitate them. Also, in the No Mercy finale, you can potentially swat survivors off the roof, killing them instantly.