1. Finger comb hair back from forehead (messy is good, so skip the comb). You want hair to go straight back because you don't want a part.
2. Pull hair into a ponytail just above the nape of your neck. Wind hair into a bun and secure with an elastic band. Pin back strays with bobby pins (have someone help you). If you want a really messy look, leave the stray hairs out.
3. Hairspray will keep the 'do in place. You can tug some strands of hair up so it's looser than the rest of hair.
Here are the classic prom hairstyle photo galleries:
Plus, check out these specialty galleries: