from my cross country days i figured out the best way to stretch my hamstring.
sit down, extend right leg, fold your left foot so the bottom touches the inside of your right thigh.
make a fist with your right hand and set it under your right knee in a vertical position (like you're holding a coffee cup).
reach over with your left hand and try to touch your toes. switch hands (while putting the other fist under your knee), then switch legs and repeat.
the fist under the knee keeps your knee from overextending, and gives a better stretch on the hamstring instead of the tendons on the back of your knee.
also, hold in the same position for at least 30 seconds and RELAX! after 30 seconds, move further and hold again. if you are only reaching for 5 seconds, you are doing nothing but tensing your muscles up.