If you're still inflexible then the only real answer is to do more.
Look up a guy called Dr Stark, he has some material on stretching and stretches which you can do. Also there is a bunch of stuff on youtube, a simple search should yield answers.
The only thing I would question is whether you're definitely certain it is hamstrings. I think unless you've had some catastrophic hamstring injury in the past which makes you certain it is them, you would do well to remember that muscles rarely become inflexible by themselves. A little more experimentation in that area may highlight some other areas of attention.