How to stretch hamstrings and prevent re-injury?

I'm a sprinter and has injured both hamstrings. 1 of them twice. And it's getting annoying because i miss big meets such as Penn Relays last year. and now im coming off a injury b4 county relays. I think im good now but im not sure. What can i do to prevent injury. how can i get better flexibility as quickly as possible for my hamstrings? thanks for ur answers!