I would personally wear that outfit with something sleek to echo the rather futuristic feel of the clothes. I’m thinking a pair of minimalist white sandals like the ones below:
Try Modcloth, Shopbop, Asos. If you’re willing to splash out a bit, Three Floor (seriously stunning and unique), Red Valentino, and Lover have amazing dresses. Instead of going for something that screams “PROM,” you can go for a pretty cocktail dress you can still wear again after that night.
Freja’s quite boyish when she dresses up. Nab her style with black skinny jeans, leather jackets, band shirts, plain gray, black, or white shirts, black long-strap bags, and tough black boots. Basically a lot of black! Add messy bedhead, and that’s you sorted.
Hmm, they sell those double-sided tape for clothing, right? I found a cosplay forum that suggests using sock glue, too. Those people seem to know what they were talking about, so you could try that. And lastly, I read somewhere that if you put a rubber band around your leg/thigh then fold your sock over it, it won’t budge! Hope one of these work for you. :)