I spent some time last night googling 1950s fashion and chatting with some of the girls who are going to this party (and who have studied this era in social history). Fashion back then was much much more conservative than today.
Trends that I noticed in photos and articles were:
*Very feminine style of dressing
*Skirts were either fitted and A-line, and generally ended below the knee.
*The hourglass shape was very much in vogue
*Tops/ Shirts were very conservative, no cleavage but still very form-fitting
*White gloves for evening wear
*Hair was generally short and curled
*Makeup was minimal during day apart from lipstick, more glamorous in the evening
So from this, I think my costume will entail of a high-waisted pencil skirt, cream top, my black shoe-boots (more for comfort rather than an allusion to 1950s shoes). I am unsure of what I will do with my hair.
I'll post pictures of my outfit tomorrow.