Here is a great infographic of a series of useful exercises which with regular practice will help avoid injury, improve your posture and maybe even rehabilitate old niggling injuries.  

I recently started incorporating this into my daily routine and its been working wonders. Frankly it would be great to see schools encouraging pupils to learn and practice these as it would do wonders for their long-term flexibility and as we know, prevention is far better than cure when it comes to poor posture.  Even if you are a relatively active person, this will work groups of muscles that don't commonly get used, so there is huge benefit for people of all levels of fitness to utilize something like this.

Posture Infographic