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 hurt 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.
Start looking out for the actual dress, now that you know what you want. Ideally, you need to start this stage at around 4 to 5 weeks before the actual event. But, remember, if you see a really nice dress in the window, don't think 'that's the one' and just go in and buy it. You need to shop around because, you might buy that dress and a week later see an even nicer one. Good things come to those who wait but the leftovers to those who procrastinate, so don't leave it too long.
Don't purchase your dress too far in advance, as before big events you can easily lose or gain weight, from stress or something else, such as periods, boyfriend troubles, stress, season (winter versus spring).