Left 4 Dead (Basic Tips on Expert)

Try and have 4 human players instead of the AI.
Also check to see if joining players have a microphone.
Communication plays an important role on this difficulty.
To check your lag push the "back" button on the controller.
Setting your captions on allows you to see special infected more clearly.
When trying to sidestep the Hunters,
try moving back and to the left or right
to strafe the Hunter till you get a clear shot or melee him.
You'll need over 50 health to effectively dodge him though.
Punching a Hunter is faster than shooting him off.
When a smoker has a survivor pinned,
melee the tongue off of him
before trying to shoot the smoker,
it's much more effective.
Friendly fire is deadly on expert,
it only takes a few shots
from any gun to incapacitate a survivor.
So be extra cautious when shooting around your friends.
Try not to run around too much as well,
Running in front of other people will almost ensure you get hit,
and most likely go down in the process.
So crouch and move slower when enemies are around.
Try mixing the melee attacks with short bursts of fire,
instead of shooting the gun in full auto, this will cut
down on how often you have to reload and still provide effective fire.
You can also reload while meleeing to protect yourself while vulnerable.
This strategy is far more effective then simply meleeing or shooting alone.
It will keep the infected off of you, save ammo, and inflict maximum damage.
When you've been incapacitated 2 times in a row
your screen should turn black and white.
Try saving your medkits for this time,
instead of healing whenever you're injured.
The reason I suggest this is because
if you go down while black and white
you'll die instantly with no chance of revival.
Using a medkit will restore your screen back to color
and give you two more times to be revived.
There are only 4 medkits per level,
so you need to ration them very carefully.
It's also important to note, there are no extra medpacks lying around.
There's only pain pills in between the safehouses.
That's why they are so important to save.
This is an annoying practice that I see people do.
Since there are only 4 med kits per chapter,
you have to share them,
don't steal someone else's
by healing yourself and then taking another pack.
You'll need teamwork, not one person with full health and a pack.
If anyone on your team has less than 50 health when they reach the safehouse
Kill them before closing the red door.
This will reset the number of times they can be revived
and they will start with 50 health,
enough to survive 2 frontal hits or 4 back hits.
Any weapons, pills, or medkits they were carrying will
still be there when the new chapter begins.
If you have the captions on, you might see the tank
before he actually attacks your group.
It's important that at least one person in your group
carry a molotov cocktail.
Light him on fire and have everyone run back.
It takes 40 seconds for him to burn to death,
whether you shoot him or not, so it's best to simply run.
If you're cornered and there is no where else to run
a last ditch effort is to hide back inside the safehouse.
Sometimes he'll break through the door and kill you,
Other times the door will hold and he'll get stuck.
I've had the best rate of success when I crouch down
and push up against the door.
If you have to confront a witch
approach her from behind and place your crosshairs
on the upper side of her back.
Most likely it'll take more than one shot to kill her.
So move back as you continue to fire at her.
If you fail to inflict enough damage fast enough
She will kill you in one hit.