How do you ease backpain relating to posture?

The back is actually made up of two main muscles on each side - the trapezius and the latissimus dorsi. So, you must be straining one of these as a result of your posture.

I actually think you answered your own question by saying the pain is related to your posture. Here is a simple answer: improve your posture! It may not be easy and it may take some time and practice, but in the end standing up tall and sitting straight in a chair with a back will help you.

Walk by a mirror, watch your posture, and practice walking straighter. It will help! For short-term relief, a heating patch may help or cream may help, but the long-term solution is to improve your posture.