First you need to know the basics
1.health packs are gold always keep at least two for the damage prone teamates.
2.Always stock up on ammo, no matter if you stocked up 5 minutes ago just do it, it could save your bacon (Oink Oink).
3.dont run pell mell to the safehouse for two main reasons:1. you are abandoning your teamates so if you get incapped later they probably will be hesitant to help you cause you were a pillock and ran off. and 2. as in the previous reason you will be swarmed so more likely to get raped by horde's/jockeys/hunters/smokers/etc.
4.keep one person with a defib in case of a botched attempt to sneak through an area on expert difficulty and that one zombie knocks you
down and kills you because THAT charger just came round the corner.
and 5. You see that crying girl? the one that looks like the girl from the ring and just as scary? No, not your girlfriend you see her then immediatly turn off your light, and try to sneak round her. if she is in sat mode (sat down on the floor) for example in the doorway to the saferoom then get your most accurate player to Cr0wn her (Shoot her and kill her with one shotgun headshot). Oh and when you get to the Mill level in the cornfield use your ears if the crying is getting louder you might want to move away.
Advanced tactics
1.when leaving the safe house have three people go out at once, the person in the middle kills the zombies as you exit while the other two cover him in case there is a zombie hidden behind a dumpster.
2.if you get pipe bombs/molotov cocktails dont just spam them like a jackass; keep them for tight areas, horde attacks and that final dash to the safe house/ rescue vehicle.
3.on the passing download content dont let the n00b pick up the M60, that is bordering on stupid because they are the first one to use all of the ammo; instead have the most disciplined players using it in short controlled bursts, otherwise the accuracy drops like a stone in water.
Finally if you find yourself in a fight with the oh-so-fugly tanks then dont run around as a group, split up, double back and get one guy as bait while the others shoot it in the back.
i hope this was satisfactory to help you survive the zombie armageddon and i take no responsibillity for the mauling of characters as this is a help guide