You have a gazillion things going through your mind when you make your way to the podium, and the absolute last thing you need to worry about is your posture.
Here is a simple tip that has worked well for me every single time.
Stand straight all the time.
Even when not speaking, stand straight with legs comfortably spread apart, arms hanging by your side in a relaxed position, shoulders nicely horizontal, and your face looking forward with a gentle smile.
Try it. It will not only improve your posture, but will also reduce the fear of public speaking and will let you focus on your message.
And by the way, it will help your aching back too!