Do you have tight hamstrings or recurring hamstring problems?
Part of the reason might be that you have underlying weakness or that you're using ineffective stretches.
Common Stretches
The most common hamstring stretch you'll see people use focuses on stretching the nerve.
This offers a small amount of hamstring stretch at the cost of a large spinal cord and sciatic nerve length.
This isn't going to give you the best hamstring stretch.
Effective Hamstring Stretches
If you don't have a sore back, try this:
Bring your shoulder to your knee with your knee bent to 90°
Gradually straighten out your leg until you feel tightness in the back of your leg
You shouldn't feel any tingling in your calf or foot as you're not doing a lot of stretching to the nerve. If you do, try the following variation.
"Thinking Man" Variation
If you experience back pain or tingling in your lower limbs, try this:
Bring your forearm to your knee with your elbow bent to 90°
Gradually straighten your leg whilst keeping a gentle bend in your spine
Look forward
These two stretches are much more effective at stretching your hamstring than the popular nerve stretch.
When to Stretch
The best time to stretch is AFTER activity.
You'll get significantly more length gains stretching after activity than before.
"Warming up" by running or rehearsing skills before exercise and then stretching afterwards is far more beneficial to your body.
Final Thoughts
If you're experiencing recurring hamstring problems or persistent hamstring tightness, using these stretches after exercising can help lengthen and alleviate tension. If these stretches cause pain, tingling or exasperate your discomfort, stop and consult your health professional before continuing.