In 18th- and 19th-century England, erect posture was considered not only a sign of enhanced femininity, but an indicator of gentility. Every gentlewoman, even if she had little income, could be distinguished by her carriage, shoulders down and back, head erect. Girls who had trouble maintaining such a posture were instructed and tutored carefully to correct their carriage. In some eras, the corset physically helped women maintain an appearance of erectness. Centuries later, that standard of femininity persists. Consider Donna Reed or even the tortured Betty Draper of the TV show, MadMen. If you are called upon to play an aristocratic lady, or even a domestic goddess of the 1960s, adopting an erect posture is one way to be convincing.
In accordance with an erect posture, an adjustment to one's way of walking may be in order. This can go one of two ways: To play a dowager or middle-to upper-class character, you will want to keep your head up, stomach in, buttocks in, and hands swinging lightly at your sides. For a showgirl, or woman of ill repute, you will want to emphasize the movement of your hips as you walk, allowing them to sway gently--or violently--from side to side. Think of Jack Lemmon in "Some Like It Hot," or Marilyn Monroe in most of her comedies.
A well-modulated voice is another sign of a dignified female person. On the other hand, a high-pitched voice, such as that adopted by Judy Holliday in the movie "Born Yesterday," bespeaks femininity as well. In either case, if your natural voice occupies a lower register, you may have to "pitch your voice up" to be convincing to an audience.
In Western society through the ages, delicacy and grace were considered the hallmarks of the well-bred female. Awkwardness and ungainliness were seen as vulgar and common, at best. Even working-class women adopted "dainty" mannerisms to make themselves more attractive to men. If your movements tend to be angular or uncoordinated, you may want to indulge in dance classes, or even hire a movement and gesture coach, to help adapt your movements to the long-held "feminine" standard.
In the West and in parts of the East, the ideal woman's facial expressions were expected to be subtle, pleasant, never violent, vulgar, or angry. A woman was meant to appear to be passive and accommodating at all times. At the other end of the spectrum, the seductive female made ample use of the pout, and other flirty, suggestive facial expressions. Practice the facial expressions that are appropriate to the type of female character you are playing, Study films of the 1930s, '40s, and '50s to discover definitive characterizations of varying feminine types. With concentration, attention to detail, and practice, you will hopefully succeed in adding touches of feminiiy to your theatre, TV or film portrayal.