You can decapitate zombies for a quicker kill. This works best with the Shotgun (the Magnum does it too, but you'll want to save your Magnum for boss battles). When a zombie comes after you, let them get close, and then point your Shotgun up and fire. This should decapitate them. However, the Shotgun does a good job at decapitating from a distance, too, which is what you should try to do if you are facing groups of zombies. Only use the pointing up method for lone zombies that you need to kill.
Note - Claire does not have any weapons that decapitate zombies.
Added by: Volke Dec 31st 2009, ID#6447