Every time a character fails to heed the obviously correct warnings/information/advice of the scientist/experienced survivor/first hand witness, and then dies because of it, drink.
Every time a character pointlessly taunts, provokes or otherwise intentionally ventures near a zombie, and is then bitten/attacked, drink.
Every time a character is trashtalking a horde of zombies ("Yeah! That's right - you want some of this? Or this? Not so tough now, are you, you freaks?") and in so doing fails to notice the zombie attacking from behind, drink.
Every time a character trips, falls, gets foot stuck in root, crevice or gets a piece of clothing/strap caught on something while being chased by a zombie (ie suddenly becomes inept at moving quickly), drink. If character is female, take two drinks.
If a character acts like a totally thankless, selfish jerk to the very people who are helping him/her, and dies as a direct result of said selfishness, take a drink. Two drinks if the character is at least partially responsible for the zombie outbreak.