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.

I also would like to add that it helps when the point man (or whoever is closest to the approaching horde) crouches when a horde of zombies is coming so those who have assault rifles can fire more freely without real risk of injuring the person in front of them. It also maybe important to note that, as mentioned in another thread, the person with the auto shotgun is usually the point man due to the large and uncontrolled firing radius of their weapon, so it is a good idea for the point man to switch places with the sniper should a horde approach from the rear; that way the auto shotgun user can keep zombies at bay with a large firing radius, the assault rifle users can mow down zombies over the head of the point man and the sniper can position his or herself between the two assaulters and pick off further off zombies. This strategy can prove to be quite effective in hallways where zombies come from only one direction.

Should there arise a situation when zombies are coming from two directions, it may be useful to split the team so that the point man and one assault rifle user are facing one way (point man in front and crouched) while the sniper and another assault rifle user face the other direction; in this situation, depending on the skill of the sniper and his or her playing style, it would be useful for the sniper, which has a lower rate of fire, to move in front of the assault rifle user to crouch and beat back zombies with the butt of the rifle (this may not work as well in Versus due to the cool-down timer) while the assaulter mows them down, or a more skillful sniper could select areas with higher concentrations of zombies so that the more powerful bullet could kill the first zombie and several behind it while the assaulter fires aswell (this, of course, can still be done from a crouching position in front of the assault rifle user so that the sniper can melee zombies while reloading).