The last such incident involved The Ship, a burning flare...and, subsequently, a lot of burning villuns. *cough 
*If any game has demanded the squad mentality, it's ZP:S. It's very difficult to be 1337 in this game without backup.
*The zombie vision ("f" key) is essential for proper targeting. It's not necessary for zombies to attack the "infected;" however, obviously shying away from a survivor will tip off his teammates.
*The flashlight is useful for identifying "turned" survivors in dark corridors. Otherwise, shut it off. The zombies notice this right away.
**This is more of a request. When spawned as survivor, do not horde weapons or ammo. It slows your rate of movement, and frustrates other players. If you know where the weapons/ammo/armor are *located, share this information with your teammates. That's something you won't find in the manual. Aside from that, strategically speaking, you do not have eight arms thereby making it impossible to...
I had to have this pointed out to me (in a polite and encouraging manner) during a game. I was in loot mode.
*"Whitey" is a valuable weapon for the zombies. He should be protected, and given the opportunity to attack/infect.
*The survivors weapons can be utilized to conserve ammo to last the round without causing too much anxiety. Pistols for long range, shotties/ARs for medium...
*There are a few melee weapons that are devastating.