"Here's looking at you, kid." These are the often-quoted words of the character Rick to his former lover Ilsa in Casablanca. When you're attending a Casablanca-style party, undoubtedly everyone will be looking at you -- and what you're wearing. To prepare, research 1940s fashion, watch the movie or a few clips, and select a character as an inspiration for your wardrobe.
If you want to dress as the Moroccan people that frequented Rick's Cafe Americain, copy the traditional Moroccan clothing.
Men: Wear a fez style hat and a long tan or cream dress shirt, sweater or robe to replicate a djellaba, which is a loose robe worn by men. Wear brown linen or khaki pants for bottoms.
Women: Use a loose, full-length, long- or half-sleeved summer-style dress to replicate a kaftan, which is a long-sleeved, full-length dress or tunic made of silk. It can be in a variety of colors -- and if it's patterned with geometrical designs, that's even better.