Your teachers were right when they told you to stand up straight and stop slouching.
Proper posture is the quickest way for you to not only look as though you have lost 10 pounds, but to project a more confident, powerful presence. Always be aware of your shoulders. Keep them pulled back and hold your head high.
Make good eye contact and remember to smile! Some people think a stern expression is powerful when a smile actually projects more confidence.
The handshake is critically important, too. The handshake is a sign of trust and a gesture of good will. It signifies openness and oneness and sets the tone of an interaction. You are judged how sincere and confident you are based on your handshake.
When you approach someone, extend your arm slightly across your chest, point your thumb up and keep your hand perpendicular to the floor. Use a firm, but not too firm, grip; shake with two to three pumps. Maintain eye contact for about four seconds. Smile.
Use a firm, enthusiastic handshake with the amount of pressure you would use if you were testing an avocado for ripeness.