Straing leg deadlifts are great for both strengthening and stretching the
hamstrings...but they can be a bit difficult to learn initially.   go to
http://www.olympus.net/personal/cablebar/RL.htm for a great description of
the romanian dead lift or to http://www.drsquat.com/articles/exer.htm for a
description of the actual stiff leg deadlift.  Tomas kurz has a great book
"Stretching Scientificly" which is not overly technical and works gret.
Most importantly stretch when yo are VERY warm.  Wear tights and sweats, get
your body temperature up and then stretch.  You get much better results when
the muscles are warm and pliable, from there one can use many different
techniques to make the stretching more productive.  There may be some free
sites around the net, I'm not sure.  Try working outside the smith machine
and really emphasize form.  the front squat (found on the same site as the
romanian deadlift article) could be a good option.  Tinker with this stuff
and let me know how it goes!
Robb