How to improve flexibility?

Hi :) I'm 14 going on 15 years and have recently been bugged by my EXTREME inflexibilty. I cannot touch my toes at all or do a split and higher level stretching makes me feel like I've pulled a heart muscle. When I was 8, I used to be part of a chinese dance troupe and was really flexible. However, I abruptly quit dancing and stopped stretching. My gymnast friend says this could have caused me to become inflexible. So yes, at this point of time I do regret quitting dance and really wish to at least gain back half the flexibilty I had last time! I really want to touch my toes and it makes me really dejected watching my friends :( Hence, any suggestions for exercises or ways to improve flexibiliy? I would really love to dance ballet again :) To all who answer, thankyou in advance :)