Hamstring stretch: Lie on back holding behind one knee with hands as shown. Begin to straighten the knee slowly until a gentle stretch is felt in the hamstring. Hold twenty seconds then let the knee bend. Repeat 3 times.
Pictures from mckinley.uiuc.edu.
This is a safe hamstring stretch as it protects the lower back and provides a stretch over the entire hamstring belly. However, it is important to be aware of each person's limitations and to recognize that not everyone will be able to straighten the leg all the way. It is very important to stretch only into light to moderate stretch, NOT a heavy stretch.