Poor posture is often the result of weak muscles and repetitive actions that encourage this poor posture. Let's look at the average persons daily activities. They sit at a desk or computer for 6-10 hours per day, then sit in their car anywhere from 30 minutes to 2 hours per day and then go home and sit on the couch for 1-3 hours every night.
Not only are they spending 15 hours a day sitting, but now they go to the gym and perform all their exercises sitting in exercise equipment! Do you see a pattern here?
Too much sitting reinforces forward head and rounded shoulders. So what can you do about it? A good first step is to make sure you incorporate some good postural strengthening exercises into your workout program.
Try these 2 stability ball exercises for better posture!
SUPINE ABDOMINAL STRETCH AND BACK EXTENSIONS 
Perform 2-3 sets of 10-15 repetitions and see your posture improve!