How do you get from a Senior Graphic Designer to an Art Director?

Management. Unfortunately no one will give you a chance as art director if you had not prove you know how to administrate others time and skills. It is not only about having "great, creative, beautiful" ideas, it is also about knowing others that are better than you and put them together under your vision to achieve that goal.Direct your photoshooting, asking illustrators to render your ideas, copywriters to polish your texts, little by little add those "project management" experiences. Take time.