Art direction and design are really two different mindsets. The best graphic designers I have known are usually not good art directors. The best art directors are not typically good designers. There are rare exceptions.
I wouldn't agree with the idea that art director is necessarily a higher ranking position than designer or that designers usually report to art directors. Advertising agencies do tend to favor art direction over design. But, at design firms, designers rule.
If you want to become an art director, you need to be skilled at coming up with concepts and directing visual elements from other artists like photographers, illustrators, production artists, etc.
If your field is advertising, there are several very good ad schools that teach art direction. If that's not an option, then you can learn from the world's best art directors by getting your hands on some quality annuals like the One Show, Communication Arts, and also the magazine, Lurzer's Archive. There are several good websites, too. Ads of the World (www.adsoftheworld.com) comes to mind.