Found this great how-to on Instructables.com, how to pull off a Decapitation Effect in your film.
There comes a time in every filmmaker's life when he needs to lop off a head. When I was a kid, I'd cut the camera and replace heads with basketballs, but this is Erik Beck from Indy Mogul we're taking out, so you know things are going to get messy. Cuddle up close to your styrofoam heads, jugs of fake blood, and stunt dummies, and enjoy. You'll need the effects from our Limb Tear, Fake Heads, Blood Shooterand episodes to do this one.