DJB is right about the Russian dancers/graduating students having their hair done for them. It was watching that in (I believe it was) Children of Theatre Street, that I saw the method of putting up very long hair in a bun. It was one bun inside another. That solved my problem for me with my long hair. I still use that method.
Gelsey Kirkland in her auto/bio talks about having her own hair stylist for the filming of the Nutcracker she did with Baryshnikov. Remeber - her hair was down.
A tricky scene is the first act Giselle, in which the ballerina dances with her hair up, but it has to come down as part of the scene. Sometimes, as Giselle embraces her mother, the mother slips out a few hair pins. I like to watch for that moment.
It isn't unusual for dancers to wear wigs. Valery Panov did when he was on stage. Sometimes when dancers need to all have the same size bun in the same place on the head - like corps de ballet in La Sylphide, Giselle, etc., they wear hair pieces.
I knew someone who danced Giselle and in the second act put light makeup where her hair was parted down the center so it would show up. It's a very classical/traditional way of doing it.
What has puzzled me is when the dancer has a small bun on each ear (very Victorian)....I tried that, but it's really hard to get those hair pins to go through the flesh of the ears...
I can't figure out how to do that. I can get the buns above or behind the ears, but not covering them.