Poise affects our posture, which affects our confidence. Small gestures like being a good listener, not slamming doors, not laughing too loudly, all help others feel at ease. ... You will gain more confidence.

When you stand, walk or sit straight, it tends to open up your abdominal cavity allowing more room for your organs to function with ease and hence, improving your overall health. ... As much as health benefits go, a good posture determines a good and healthy mind as well.Here are some of her etiquette tips to try...

Posture & Poise:

When the Duchess crosses her legs, it is by the knee. She keeps her ankles and knees together at all times and either crosses at the ankle into the “Cambridge Cross” or does the “Duchess Slant” which is a coin termed by Myka Meier to identify a method long practised by Princess Diana and The Duchess of Cambridge. Typically ‘the Duchess Slant’ is used when a lady has to sit for an extended amount of time while keeping poise and posture. The key to the technique is to square your shoulders straight ahead while maintaining perfect posture. Keeping knees and ankles together at all times, position your legs so that you create a slant, angling your knees to the side. Hands should be folded one over the other and placed in your lap. It is the perfect pose for when a camera is shooting directly in front of you because by slightly slanting the knees to create a zig-zag effect when wearing a dress or skirt, your legs are angled so that the camera only shoots the sides of your legs and protects your modesty.

What should you do with your hands when you don’t have anything in them? Put the webs of your hands together and in front of you for a “resting” poise or lay one on top of the other. 
It’s all about Grace! To appear more graceful… simply slow down, both your movements and the speed of your voice.

Posture:
When seated, shoulders are rolled back. Do not use armrests and sit about 2 inches away from the back of the chair and 2 hands away from the table. Never lean back into your chair, as your spine automatically curves and you break posture! 

Another important posture cornerstone is your chin. Like the Duchess, your chin should be parallel to the ground level. If it is too high, you may appear uninterested, or too low may show lack of confidence. Remember to gesture instead of pointing.

Posture while walking, climbing and descending stairs - do not look down at the step after the first one. Your chin should be lifted slightly and eyes should remain ahead of you. Do not grasp the staircase railing but instead gently put your hand on top of the railing to guide yourself. Toes should be pointed toward the staircase wall and be sure to walk very slowly upwards and downwards.

Dining Tips:

Napkins - Cloth napkins should be folded in half and the crease should face toward you. When you need to use your napkin, open the right-hand corner of the napkin and dab your mouth…then stains always stay inside of the napkin and never on the visual side.

No chewing ice or gum! 

Never announce you must use the restroom, simply excuse yourself.

No lipstick or makeup application at the table, or anywhere in public.

Quite possibly the most important etiquette tip - Having etiquette like the Duchess simply means being kind, thoughtful, respectful and gracious to everyone and all living things around you.