The neck is especially important in contact sports such as football and rugby, where neck strength in all planes is highly valuable in preventing injuries that may result from collisions and violent jerking of the neck. Neck harnesses, manual resistance, and even four-way neck machines are all good bets along these lines, as training the neck can be somewhat awkward.
From a postural standpoint, specific work for the neck flexors is an effective means of correcting forward head posture when paired with stretches for the levator scapulae and upper traps as well as specific interventions to reduce postural abnormalities at the scapulae, humeri, and thoracic spine. In this regard, unweighted chin tucks for high reps throughout the day are all that one really needs. This is a small training price to pay when you consider that forward head posture has been linked with chronic headaches.