For example, style your hair with an extreme side part and Veronica Lake-esque waves. Or, wear it in a dramatically short style like Jean Harlow. For inspiration, study pictures of your favorite actresses on the red carpet or choose a film noir icon to find that perfect coiff to "wow" everyone with.
Try not to reveal your entire face. Let your curls or waves fall over the side of your face to keep some of your features shrouded and mysterious.
Try an A-line cut for a bold and enticing look.
If you want to model yourself off of a vintage femme fatale, stick to true, bold hair colors, like platinum blonde, dark brown, black, or auburn.