Posture is an important part of movement, meaning with proper posture we can avoid compensation patterns and a lot of pain. With poor posture in the upper back, shoulders and neck we can end up with shoulder pain, neck pain, headaches and more. In the more extreme cases we can end up with Kyphosis; which is the rounding of the upper spine, think of when you have seen an elderly person all bent over, not a very comfortable look.

Front View: Left  overactive traps, Right rolled shoulders. This may look excessive but I have had several clients walk in looking like the first photo. Second Proper posture, with relaxed shoulder and relaxed traps. 
Lateral View: Left Rolled shoulders, Right overactive traps. This view really shows both issues fully, with the rolled shoulders is also a forward head poke. Second photo, shoulders are back neck is in line and shoulder relaxed, looks so much better!!
Lets get started, what can we do to combat this: 
1. Foam Roll & Trigger Point. Lets loosen off those tight muscles to start off with. Problematic areas: Pec Major, Lats, and Upper Traps 

2. Stretch it out. Grab a foam roller and place it on the spine, practice opening arms out to lay beside you and up over head. Hold each position for a few minutes, try to ensure low back isn`t arching, in one of the photos you can mine is starting to. 
Stretch out the neck by placing a rolled up small towel under neck and laying on it.  Hold for a few minutes. This really is the easiest part of any workout.
Another great stretch to start a workout with is wall slides, ensure your heels, bum, shoulders and arms are touching the wall and slide them up to the point they are still in contact with the wall. Do not arch low back! As we can see my range of motion is limited as my arms shoulder go all the way up. This is why these are in my program too.