I hate stretching my hamstrings. It's way too easy to strain them. I mean, how do most people do them, anyway? They stiffen their legs dead straight, then try to bend over and touch the floor. Of course, there's a good chance they can't touch the floor, so they start bouncing. OUCH!
I know a lot of people say that's how you increase your range of motion. They're wrong; that's a good way to injure yourself. Remember what I said a couple of days ago about the nervous system and how it protects your muscles by limiting your range of motion? Bouncing is an attempt to overcome that protection. Bad mojo, mon!
Let me teach you a hamstring stretch that's less likely to cause damage.
Spread your feet about 24 inches apart, with your toes pointed straight ahead. Then bend forward and place your hands on the floor, maybe 10-12 inches apart and maybe a little farther in front of your toes. You might only be able to put your fingers on the floor in this position; that's ok. And yes, you will bend your knees to do this! Bend your knees as much as necessary to get in this position without strain. You'll probably find that it feels better if you stand with your weight on the insides of both feet; that makes your knees and legs form a straight line between your feet and your hips.
If you've done this, your upper body is supported mainly by your arms, rather than your hamstrings. Now all you have to do is straighten your legs.
It's not easy, is it? Don't worry if you can't do it; just straighten them until you feel a little stretch and hold it for a few seconds, then relax. Your upper body may have to move a bit forward when you do it, and that's ok too.
And here's how you adjust the difficulty: Widening your stance makes the stretch a little easier, narrowing it makes it more difficult. I suggest 24 inches as a starting point simply because most people can spread that wide without difficulty, but if you can, you may want to start with your feet further apart. (Personally, I find about 30-32 inches to be easier.) As you get more flexible, you may eventually get to the point where you can do this with your feet together.
But if you don't, that's ok; the goal is just to become more flexible than you are now, not reach some pinnacle of pretzelitude.
This is the least damaging way to stretch hamstrings I know. Of course, all medical disclaimers apply; try this at your own risk. (But I still think it's less risky than any other method I've seen.)