Stand up straight, sit straight.
Models say, when standing push breasts out, suck tummy in and push buttocks out.
Also, do not slump. When sitting and talking to someone, concentrate on listening, don t change subject, do sit forward in an attentive position. Do not make big movements, instead take small steps, walk on curbs not over them, as found in parking lots. Carry your weight forward. High heels will automatically push your calves out.