Before you call the helicopter, look for a building with a satellite dish on it.

Go up the stairs next to it and continue to melee the door that it leads to

Note: Contrary to what people may say, it CAN be broken by the player, it just takes awhile, even with all four players.

Go inside and hug the back wall.

Zombies will never come in, except in rare cases (in which case it is just a Common Infected).