Ok...so you may not exactly survive... but you will likely score better than the other guys.

Survivors

• Sloth is your enemy. The zombies have spawn timers, and every second you waste dilly-dallying in the same spot, the more human-controlled special zombies you're going to have to face. Keep moving quickly, but at the same time, stick together.
• As an addition to the above, plan ahead. In the second level of Blood Harvest, my team, as zombies, completely obliterated the Survivors in the emergency exit room because they waited too long to open the door.
• Don't try to hide. The zombies can always see you. Instead, stay in the open and close to your friends so that they can help you if a smoker or hunter strikes.
• People like to hide in trees and thick bushes in Blood Harvest. If you suspect somebody's camping in foliage, such as an audio cue, don't be afraid to open up on it with your pistol. They already know where you are, so you're not giving your position away.
• Don't let things spiral out of control. Think of here and now. If your friend is pinned by a hunter or caught by a smoker, take care of it. The more you let the zombies work together, the more likely you lose.
• Don't be afraid to make a run for the safehouse if you're close, just be on the lookout for ambushes. Once that door's shut, not even the Tank can get you. Every person who makes the safehouse increases a multiplier bonus by 1, so even if only two of you make it, your round score still doubles.

Zombies

• Coordinate your attacks. Attacking one by one is simply asking to die as you slowly peck away at their health. Typically the best way to coordinate is to induce the maximum amount of chaos possible as soon as possible. Typically first you want your Boomer (or Tank if you have one) to attack, making the enemy focus on them, and hopefully either attracting more zombies as a boomer (inducing chaos) or getting them all to focus on the tank while other zombie types attack from behind.
• If all but one enemy is incapacitated, wait until the one guy tries to revive one of his friends, then attack. He'll have to notice, which he's probably looking at his friend, and he'll have to react, which will be delayed due to his attention being elsewhere.
• Set up ambushes. The Survivors are set up for direct attacks. The zombies are not. You try to rush as anything other than a Tank and you'll find yourself a bloody piece of Swiss cheese. Hide in side rooms, foliage, you name it. Don't forget that behind is a great place to take somebody out unless they're actually watching back there.
• Try to attack from angles where you can only see one Survivor, especially as Smoker. This prevents them from instantly mutilating you, lending you time to do damage.
• As Smoker, try to get relatively close. Sure, getting dragged up the side of a three-story building is cinematic, but that gives them plenty of time to shoot you.
• Smokers are good at trapping a player in a horde rush. This is especially good when they're tagged by a boomer. They can't fight back, there's a crapload of zombies on them, and best of all, a buffer zone of zombies between you and the other survivors if you've positioned yourself well.
• As Tank, do not get carried away at killing every enemy like the AI tank does. The more you incapacitate the better, as incapacitating all of them means you stop them where they are.

Last Word

Communicate! Use the voice chat, coordinate with your team, no matter which side you're on.