Your posture can entirely change how you look AND how you feel. I don't know a soul who doesn't want to look and feel his / her best, so this is an important topic to touch on. There are many complex articles on posture, so I've tried to simplify a few tips for accessibility to get people practicing these habits regularly.
Do you sit most of the day? Yeah, many of us do. Even as a trainer, I am smushed (good word, right?) in the seat of my car getting from place to place for at least half of the days, sitting for at least a couple of hours a day doing desk work, and so even my posture can suffer despite working to counter it in my bodywork. If you add in shallow breathing (using the shoulders and neck muscles over the diaphragm) that is symptomatic of always rushing, it can be a recipe for an anatomical catastrophe. This is an example of where a few mind tricks and some awareness can dramatically change our bodies.
Diaphragmatic breathing, breathing in which you really focus on expanding and emptying the lungs (by contracting the diaphragm to make space for the lungs), lifting the ribcage rather than the shoulders, reduces stress hormones and relieves tension from the neck and shoulders. In order to get a feel for this, begin first lying on your back, with your shoulders pulled away from your ears and your knees bent. Place one hand on your heart, and one on your diaphragm (just below the ribcage in the middle of your trunk). You want to feel a deep rise and fall here as you breathe, and to keep the shoulders still. Once comfortable with this, you can easily tap into it sitting or standing as well. Just as this breathing will help correct posture on its own, poor posture can inhibit proper breathing. So the two must be considered as a unit.
I know everybody says this, but I'm going to say it again: Try to get up and walk around as often as you can throughout the day. The lungs and the heart want to work together to get oxygen pumping through your blood to all areas of your body. Be cognizant of placing weight into your whole foot when you stand, letting your ribcage expand and fall when you breathe (this will lend itself to an active core all on its own) and keeping your shoulders down, and stacked over the pelvis.
* have your body weight pressing evenly into your feet, including the heels
* use diaphragmatic breathing
* keep the shoulders relaxed, and stacked over the pelvis
Having a program to counter rounded seated posture, as well as one that benefits your neurological system and circulatory system is imperative for all human beings these days. But at the very least, bear these pointers in mind and I guarantee you by the end of day one, you will feel better and healthier already.