This is how you can stand up, tall and confident. You know the Hunchback of Notre Dame? Yeah no one wants to look like him, (even though he was a sweetheart).

Keep your shoulders rolled back and down. By keeping them in this position, you automatically increase the stability.
Bring your chest up, pull in your abs and try not to arch your back.
This will give you an air of confidence plus it will help offset the weight shift from wearing heels.
Keep your head (don’t let your chin jut out), shoulders, hips and knees equally lined. These are the building blocks of proper alignment in any position and make you look slimmest.
The best position is to distribute your weight between both feet and place your hands behind your back. But that does look a bit too military so the best fix is to simply shift your weight between your feet instead. It may not be the best for you but it does help.
Pictures from Seventeen magazine