HOW: Lay down on an elevated surface like a bench, a couch, or a bed. Bring one leg off and march that leg up as far as you feel comfortable. The lower the surface the more aggressive the stretch will be. Reach back for the ankle of the leg on the elevated surface and bend you knee as far as you can tolerate. If you can't reach your ankle then use a rope or towel to help maneuver the amount of knee bending you go through.
FEEL: You will feel the front of the thigh that is on the elevated surface.
COMPENSATION: Avoid arching at the low back with this exercise.