You have to question why your hamstrings are so tight in the first place. More often than not the hamstrings become tight because they are overworking and compensating for weakness in the core and the glute muscles.
If the glutes are not functioning as they should then the hamstrings will work instead. They will also shorten with certain posture types. I would therefore suggest retraining the core and the glutes and improving your posture with our clinical Pilates exercises the hamstrings will naturally lengthen with improving the function of your core and glutes and improving your posture. I can speak from personal experience!
When stretching any muscle I recommend holding a stretch for 30 seconds and repeating five times to actually change the muscle fibre length. I also find that including a stretch for the fascia (connective tissue that surrounds the muscle) is a much more effective way of stretching.
The two stretches I recommend to my patients with tight hamstrings are a standing forward hip pivot and a downward dog stretch. They are very active stretches and can actually improve your hip stability at the same time. Both stretches are included in our clinical Pilates workouts