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Short neck :D (and saggy skin)
Tilt the head to the side you are shaving and pull the cheek skin up. That will give a nice smooth surface to shave your neck area

When going side to side on the neck, I try to pull my neck skin up to the cheek area. It makes the stroke a lot easier and covers about 3/4 of the neck. The lower 1/4 only gets a two-pass shave...