With small peach fuzzies, I use the 'flick' I flick the back of the head hard about 3 or 4 times in quick succession, holding the mouse in my left hand. It kills them straight away, but they twitch for a little while after (Only way to get Lil to convert from live pinks)
With bigger mice I find a very sharp blow to the back of the head to stun then breaking the neck with a set of forceps works for me. I read about someone having the tail de-glove or break off when they pulled on it so I haven't tried that method of cervical dislocation