You really need a posing coach, because it is almost impossible to explain that with words - you need someone who will be able to show it to you personally. Trust me - this would be the fastest and the most efficient way of doing this.
Try to roll your shoulder blades up and forward. Practice practice practice. Eventually you will get it.
Set up some mirrors around your house and practice regularly. Posing is very important and something a lot of competitors neglect. Remember that the judges can only judge what they can see!