The neck rotation is a standard stretch in most martial arts. It's easy to perform, can be done seated or standing, and requires no equipment - in short, everything you want in a stretch.
Regularly performing the neck stretch will help to keep this area flexible. This is important - our modern lifestyle places great demands on our bodies, forcing us to sit in unnatural positions for hours at a time.
Whether it's driving or sitting at a computer, holding your head upright for long periods of time can cause tightness in this area. Left untreated, this can cause pain.
Performing the stretch is easy. By rotating the neck from side to side, you stretch out the three main muscles - the splenius and levator scapulae on one side and the opposing sternocleidomastoid.
Either sitting or standing, turn the head to the side, as though you are looking over your shoulder, and ensuring that you don't raise or lower your head. You can use the hand to press against the side of your head and create more resistance.
As with all stretches, go to the point of mild discomfort, but not pain. Hold for 10-20 seconds, relax, and repeat three times each side.