Always, always, always stick together.
This is the cardinal rule of Left 4 Dead. Never venture away from your team unless there is only one entrance into where you are going, no way for zombies to drop from the roof, and your buddies are guarding said only entrance. Running ahead of your team is a great way to get torn apart by a special infected such as Hunter or Smoker before your teammates can ever reach you. People that just run haphazardly away from their team get strangled, torn apart, and covered in bile. The same happens to those that lag behind for whatever reason, but make sure to stick with your slowest team member. All challenges are easy when confronted by a team of 4 coordinated players.

Pace yourself to the tempo of the game.
This is the one rule that almost nobody follows, and it's a shame. Left 4 Dead has a very distinctive tempo associated with it. Those teams that move too fast end up drawing groups of zombies with them, endanger each other with multiple attacking special zombies, or trigger witches without realizing it. Conversely, if a team sits around in an area too long, the AI director will spawn increasingly harder waves to get them to move forward. It'll even spawn Tanks! The base tempo of the game is that you have time to do a quick sweep of each room along the main path and kill the zombies inside. If you use this tempo to pace yourself, you'll find that Left 4 Dead is challenging without being impossible!

Get upgraded weapons at every possible opportunity.
You may like your Uzi or pump-action shotgun, but let's face it: they suck. When you're fighting zombies, you want some more firepower. That's where the advanced weapons come in handy. There are three weapons to choose from: the assault rifle, hunting rifle, and the semi-automatic shotgun. These weapons appear at random spots throughout the maps, but the random spots are always in particular locations, so play through multiple times and learn! There are also set areas where they spawn, such as in the safehouse of certain episodes.

The assault rifle is great for large groups of enemies thanks to the high rate of fire and relatively good accuracy. This makes it good for the two people in the middle of a group. Just point at the headline of a mob and spray! The hunting rifle is the only weapon in the entire game with a scope, making it perfect for picking off those nasty long-range zombies before they get anywhere close to you. It's also extremely accurate, a one-hit kill, and travels through bodies, which makes it the weapon of choice for the guy in the rear. The semi-automatic shotgun is the best weapon for the guy in front due to its very high damage output, large amount of ammo, and spread. It's also the best weapon for taking down a Tank, as it can deal more damage in a faster period of time than any other weapon.

Learn to hide and cover fire.
This is not Half-Life 2, nor is it Counter-Strike. It's less realistic than Counter-Strike, but more realistic than Half-Life 2. Unlike either game, though, the nature of Left 4 Dead promotes the usage of smart tactics. Learn how to fire over your comrade's heads without injuring them. Find the optimal configuration of survivors for a particular scenario. Learn the hiding spots that let you funnel hordes into killing zones, like subway cars, air vents, and doorways. You can't leave your teammates alone and you certainly have to be smart about using the environment to your advantage. Zombies come from anywhere and everywhere. Be observant.

Take out special zombies first.
This seems like a no-brainer, but it's a rule many people forget. If a special zombie is in the area, take them out first! Special zombies are a huge drain on your resources if they manage to snag you in a trap, but if you take them out quickly, you don't have to deal with them on top of a group of common infected. Also, the sounds a special zombie makes can give away its position, and tell you what kind of special zombie is nearby. Also, if you are close enough to hear the sounds relatively well, a character will identify the special zombie for you. The sounds are:

• Hunter: Growls and snarls. When it leaps, it makes a distinctive screaming sound that sounds like several normal screams overlaid on each other.
• Smoker: Coughs and wheezes. When it launches its tongue, a slurping sound is made, alerting you to its direct position. Following the discarded tongue will lead you to a smoker as well.
  
• Boomer: Belches and groans. When it barfs bile at the survivors, it makes a terrible vomiting sound.
• Tank: Grunts and snorts. The Tank is hard to miss sound wise, because the music gets very intense when he is around and he bellows like an elephant.
• Witch: Crying and sobbing. The Witch is also very hard to miss as the music becomes creepy and choir-like when she is nearby. The louder the music, the closer she is.