What are some good stretches / exercises to help build up muscles in my legs for ballet?

I was wondering if anyone knew any good stretches / exercises to help build up muscles in my legs for ballet. I can do the splits every way and can sit in them comfortably for periods of time. When I do grand rond de jambe, grand battement etc I can never get my legs up very high. What are some good stretches / exercises to help get my legs higher? thanks.