Hamstring muscles are tough to stretch.  This exercise is a great way to warm them before a training walk.

 Steps:

1. Lean against a wall or tree.
2. While facing forwards, swing your leg forward and up, then back and around, as if it was clearing a hurdle placed at your side.
3. Use a prop such as someone's arm to act as the hurdle if possible.

Figure A

Figure B

Figure C

4. Range of motion is important to performing this drill correctly.  Be sure to extend your leg in as large a circle as possible.
5. Perform this exercise 10 times for each leg.