It’s probably one of the only times you could actually get away with dressing up like a princess without looking ridiculos.
Still you should not exaggerate! Don’t add tiaras or many jewelry items. Don’t get too fancy with the hairdo or too much make-up and glitter. If you get a big dress, go with a simple hairdo, maybe a braid and a more natural make-up. And the other way around. If the dress is simple , without any embellishments, you can get a fancy clutch, a statement necklace or a fancy hairdo, but not all at once. Try to focus only on one detail.
Pastel shades will always be trendy because they have a certain feminity that can not be replaced. Red, black and nude are also timeless, but you need to get a simple, A-line dress to get the clasic look . Spend you money wise, go shopping online for the best deal and get your prom dress for the best price. Try styles in your local shops and find what look best on you, then search for it online and get it! Beaded details will look great but try not to go very bling bling. Crystals may look good in pictures, but in the real life they might be too much and you don;t want that.