In the days of old, the ballet was a red-carpet affair that called for gowns and suits. Contemporary culture no longer requires such extravagant garb, but dressing casually for an event of the fine arts can be even more challenging. Don't forget the three C's of fashion: wear something comfortable, look classy, and pick something cute!
First and foremost is to wear something comfortable. Although the ballet is a wonderful experience, it is a long one. Theaters are often significantly air-conditioned to balance the body heat of hundreds of people gathered in one room, even in the winter months. Bring a sweater or shawl; you don't want to be distracted from the performance because you're shivering!
One great option for the ballet is to wear a skirt or dress paired with heels or flats and a sweater. Avoid wearing too-short skirts, shorts, jeans, or tattered clothing. You can dress in your personal style while also showing respect for the dancers and the music. Since the ballet is also the perfect date or family outing, you also want to keep in mind how your companions will be affected by your outfit. Another way to think about going to the ballet is that you will likely go out for dinner before or after, so you will want to dress appropriately for a nice restaurant, too.
The ballet is a great opportunity to experience the fine arts. Of course, it's easy to look great while you're having fun!