@rundocuri- Walking in a pattern that consists of wide strides also help to loosen up the hamstring muscles. You can do this by starting out at a slow pace then increasing your strides. If you have an area to walk that has hills, walking up them will also provide a great workout for your hamstrings.
Once you start flexing your hamstring muscles with walking exercises, you will be ready to begin an exercise regimen that includes weight training. This type of program will go a step further to not only keep your hamstrings flexible, but to also create muscle strength and definition.