Most Helpful Girl
I think it sounds beautiful! :)
You should wear diamond stud earrings to go with your headband, no necklace, maybe a diamond bracelete. (all the diamonds around the same size).
I think you should wear your hair down, but that's just me. Cause your dress is long, right? So it'd look nice with hair worn down, but that's up to you :)
Make up... I think maybe black eyeliner, but don't overdo it, you don't want to look like a raccoon. Maybe a bit of really dark blue eyeshadow and a bit of grey (but once again, don't overdo it... and Urban Decay [brand] has a really nice dark blue color if you don't have one!). Then wear lots of mascara :)
Also! Its prom! so do whatever YOU want to do with your hair, make up, etc. honestly, it'll make you feel better if you go by what you want :) I'm just giving suggestions.
I hope I helped.