One method I heard is to tie a string around its feet and hang it upside down. It will fall asleep. Then quickly take a screwdriver, through the beak, punch it through the skull and twist. If you do it just right, they don't even have time to ruffle their feathers. Supposedly this helps make plucking the feathers a whole lot easier to do to if you ever want to grow chickens for food.
I will have to look around and find the site that suggested this method...
EDIT:
Here it is! The Rouge Turtle: Slaughtering Chicken