I think ’50s style is all about the silhouette. Clothes with a slim waist and a flared skirt can look kinda ’50s-esque even if the other elements of the outfit are more modern or from another era. Shop for “fit ‘n’ flare” dresses as well as high-waisted A-line skirts.
Go to a MAC store (or the makeup purveyor of your choice, though I’ve always had excellent experiences with MAC) and ask them to help you find your perfect red lipstick. My favorites are MAC Russian Red, MAC Heartless, and Kat Von D Outlaw, but different skin tones call for different shades of red. You should also ideally pick up a black liquid eyeliner while you’re there – it’s vital to the look!
When I want to dress rockabilly but don’t feel like looking too fancy, sometimes I wear well-fitting jeans that are rolled up at the bottom, and a cute button-up blouse.
Many people would tell you that a good pair of heels is crucial for a ’50s look but I don’t think they’re a strict necessity. They might help you get into the rockabilly mindset better, though.