Once you have narrowed your choices to one particular prom dress style, try on that style in a variety of different colors and finishes. Although you should already have an idea of the preferred color, it can't help to do a final check for a glamorous shade that brings out color in your face.
When choosing a fabric finish, keep in mind that shinier finishes will tend to show off figure flaws while matte finishes will obscure and diminish undesirable features.
Then try on dresses in a variety of different styles and cuts to find the one that works best for your body. You should already have done the work on the body shape, but here are some additional considerations:
If you have a slim figure, you may look best in a sheath-style dress that shows off your silhouette.
If you're short and petite, dress shopping may be more challenging; cocktail dresses become full gowns and party dresses become cocktail dresses. However, with plenty of time ahead, you can get these tailored to your size.