Posture is really important. It changes the way energy flows through your body, it changes your mindset and psychological well-being and it changes the way people view and react around you. Most of us (irony noted) spend hours and hours sitting at a computer. Deltoids (shoulders) wound forward, pectorals (chest) slightly closed, typing away. The shoulder blades are expanded and released. Too much time spent in this position leads to weakness in the muscles beneath the scapula in the back (sub-scapular muscles) and tightness in front. Also, most of the motions we humans engage in are focused in front of our bodies, therefore the back becomes neglected. It's important to make up for this, with proper training, to stand up straight and use our anatomies properly!
Every time you train your back (with exercises such as a seated row or a lat pulldown - and there are hundreds of exciting variations of these exercises), you need to set your shoulders first. My clients reading this are laughing. "Set your shoulders" is probably a phrase I say the most, while training. Its direction is to protract the shoulder blades, pulling your shoulders down and back, away from your ears. This expands the chest, and allows for proper closure in the back. You MUST be expanded on one side, in order to properly close / contract the opposite side. Makes sense, right? If you don't set your shoulders at the beginning of EACH REPETITION, you will be ripping yourself off of the most beautiful contraction the world has ever seen. You will stay slightly shut in front, and won't reach that deep, tight closure in between the shoulders blades.
Please, don't swing your body while training your back. Oh how that pains me. Keep the core tightly gripped, so that the proper muscles can take on all the range of motion that they are meant to, in isolation. Freeze the core, set the shoulders, exhale and allow the subscapular root muscles to take on all the load. Poof - after a few weeks / month, you will be standing straighter and enjoying life in a whole different way. Perhaps I sound like I'm dramatizing the effects - just try it. You'll see.