Does anyone know any good posture and exercises for the back to stand straight for the ballet?
I take ballet classes four times week, more modern. My teachers told me that I'm stuck in my hips and I need to go further. I think I'm arch my back. Is anyone know of useful sites and / or exercises? Thank you! 
Here is a simple exercise, but it'll help charges lie on the floor, trying to feel all them from your back, your legs and arms too, but mostly back on the ground. after you get used to this position, try to make your movements while lying on the ground like that, pretend you are standing. and when you're practicing ballet, imagine that there is a glue stick back and try feel your back touching the stick your imagination as you did with the floor. Another thing is useful to imagine your head is pulled up with string. Hopefully these charges will help! =))
Exercises for Posture - Stabilizing the Lower Back