Some things simply go together. Things like peanut butter and jelly, baseball and hot dogs and the prom and long hair. Okay, maybe you think that is a stretch but if you take the time to look at how many more options you have in hair styles when you are trying to do it with long hair as opposed to short hear, you will quickly see what I am talking about.
With short hair you are limited in what can be accomplished. Long hair just styles nicer when you are trying to get it to flow well with a long prom dress and the accessories that go with that dress. You can go traditional and let your locks flow straight and long much like country star Crystal Gayle or you can have it as old fashioned as a traditional bun wrapped up and piled on top. There is no right or wrong option. It is whatever goes well with the outfit and your personal tastes.
This season it looks like having the hair loosely braided and then wrapped around the head in layers and pinned has become very much in vogue and with a flowing pastel dress it gives the aura of a princess in a far away land and can make the young woman look very sleek and regal with a fairly small amount of effort.
Also smaller groups of curls, almost resembling dreads are popular and will give a rather rough looking edge to the softer material and design of the prom dress its self. One of the main factors in determining the final design of your hair style will likely be the overall texture of your hair. Fine hair lends its self more to the many options available than does the more dense and coarse hair that a lot of younger woman have.
Here are some pictures of prom hairstyles for long hair, enjoy.