Good posture combined with body tension is one of the keys to success in gymnastics.
Try using your body body tension when you stand. Fight against sagging through your lower back so that your tummy pokes out (a favourite stance for little girls!), try instead to stand tall.
Use your muscles to straighten your back and lift your head and see if you can make yourself just a little taller. The front of your hips should feel flat. When you stand up nice and tall, don’t lift your shoulders – they should be relaxed.
A little effort will go a long way!