Posture is important to a hypnotist because if the body is not properly aligned the voice can't project with strength, tone, and leverage that iis required.
Imagine there is a thread through the crown of your head, neck, and down through your spine. Now imagine someone pulling up on this thread, aligning the neck and the backbone to the degree that if there were no muscles holding them in place the vertebrae would stay sitting one on top of the other. You should simultaneously feel your shoulders slide back into place.
Practice this posture while standing, working at the computers, driving, etc... until it becomes second nature. This will allow your voice to project properly, as well as give you good posture ;)
You may find stretching out the shoulders and chest will help as well.