Poodle skirts with lots of petticoats and black & white saddle shoes are what really marked the 50's. If you don't have that, you might also go for jeans rolled up to just below the knee, a white over sized men's button-down shirt, and white bobby socks (the type that fold down) and white Keds sneakers. A scarf around your neck or tied in your hair adds to the look, so would cat-eye glasses if you can find them. Or a satin jacket like the girls wore in the movie "Grease".
I have the whole poodle skirt outfit, complete with petticoats, black & white saddle shoes, scarf and Cat-eye glasses that I bought for Halloween a few years ago. I ordered it from the Bowling Shirt website. Not cheap (over $100), but the whole look is complete and good quality clothing.
http://www.bowlingshirt.com/poodle/outfits/