Strengthen the Lower Trapezius, combined with a squat. Be careful to keep the neck soft and avoid over extending it. The lower portion of the trapezius holds the head up. Keep it strong and you can prevent a lot of neck pain.
Sidebend over a ball, with feet anchored against a wall. You will strengthen the Quadratus Lumborum, Tensor Fascia Lata and (to a lesser degree) the Abdominal Obliques (Transversalis). This move is using the tensor in its functional capacity as a pelvic stabilizer and the quadratus in its capacity as a mobilizer.