I prefer to warm up a bit before stretching, especially hammies. 5-10 minutes or so spinning lightly @ 90 on the trainer is a nice blood mover.
I prefer the standing hamstring stretch.
Standing Hamstring Stretch: Flex your left hip to 90 degrees, placing your left leg over the top of a chair or table (or whatever) Bending at the waist, lean forward, reaching for your left shin or toes until you feel a stretch on the back of your thigh. Hold for a count of 10-15 seconds, and repeat 3-5 times. Repeat with your right leg.
Keeping your back nice and flat is another key for me with hamstring stretches.
Deep Breathing is another key for me.