A great coaching cue that I like to use is to tell people to lift up their index finger as they perform a set of pull ups. Not only does this put the shoulders into a better position but it will also make it infinitely harder for someone to power through with their biceps, which in turn will give you better at pull-ups and stronger, bigger lats.

Most people that experience shoulder/arm/neck issues struggle to match their normal max set of pull-ups with the index finger lifted, so this is a great way to check if you've been using the correct muscles. If you’re only one or two reps off that's fine, but if you find your reps are cut in half or you really struggle to do one, then there's room for improvement! – Tom Morrison