Let's face it - not very many people have very good, let alone perfect posture. Often this becomes even more apparent when you stand over a golf ball in the setup position. Bad posture can cause many problems in the golf swing such as lessening potential distance (power), direction of the shot and the quality of contact.
In golf, good posture generally means that the spine is straight. A golfer must bend over to hit a golf ball but it is very important that this bend occurs at the waist and not by creating an arch in the back.
In order to ensure, or at least help you straighten your spine (or back) during the setup is to stick out your butt and tilt your head upwards slightly so you are looking down your nose at the ball.