I've been doing a really effective hamstring stretch lately -- you need a length of stretchy material or something to use, though.

At home, lie on the floor with one leg in the air. Loop the cord/stretchy material/whatever around your foot and pull, being careful to keep your knee mostly straight (but not locked). Hold 30s.

It really seems to isolate the hamstring, whereas other stretches I've done seem to stretch the back as much as the hamstring.