How do I work on flexibility?

Ok so I'm a runner xc, 10k, halfs, whatever and also run track 500m and 1000m. I'm pretty good and am trying to take it to the next level. I have great speed but im very inflexible so it's harder for me to stay in front after the first bit. So what stretches etc. can I do to increase my flexibility? I can hardly touch my toes people and it hurts really bad when I do. Any suggestions are appreciated.