Standing Forward Bend
One of the things people typically say before they've tried a yoga class is that they are inflexible and they can't touch their toes. Just to dispel a myth right off the bat, you are not required to be flexible nor touch your toes when you begin practicing yoga. One of the wonderful benefits of yoga is that you'll gain flexibility as you continue to practice.
If you're wanting a yoga pose that will help you with tight hamstrings, then you'll want to practice Standing Forward Bend. The name of the pose states exactly what you're doing with your body; you are folding forward, bringing your hands down toward your toes. It seems rather simple, but there are ways to effectively and safely move into the posture to gain the full benefits.
Start by standing nice and tall. Feel the lengthening in your spine, keep your gaze forward, roll your shoulders back, engage your core muscles, and feel your feet securely planted into the ground. Essentially, this is Mountain Pose, a yoga posture that often begins a yoga practice. It is also the opening posture for the traditional Sun Salutation, a sequence of poses that flow together. One of the first poses in this sequence is actually Standing Forward Bend.
Inhale and extend your arms straight up into the air. This will increase that lengthening effect through your spine and your entire body. With a slight bend in your knees, exhale, and slowly fold forward, hinging at the hips with a flat back. (Putting a bend in the knees protects your back as you fold forward.) It's ok, though, that your hands do not reach the floor or your toes. If you're new to the pose, it's perfectly fine to maintain a bend in your knees and only bring your hands to your shins. Remember, postures of any kind take some time to fully develop, including this one. Feel free to stay in this folded position for several breaths so that your body, particularly your hamstrings, can get used to the stretching sensation.
Here's a few other things you can do while you're in this pose to make it more effective. Whether your hands reach your toes or are resting on your shins, lean forward a bit so that you shift your body weight towards your toes. At the same time, feel as if you're hips are lifting upward toward the ceiling. (Your knees can remain bent throughout this process.) By doing this, you create a mild lengthening sensation down your legs. You may notice a stretching feeling in your hamstrings as well as your lower gluteus muscles and even in your calf muscles.
Further, inhale and attempt to elongate through your spine. (All you really need to do is lift your head slightly and try to extend forward.) Exhale, draw your navel in toward your spine, and lower yourself further toward your legs. Now, both of these things (lengthening your spine and going lower into the pose) are very subtle motions. You don't have to expect big movements to go deeper into Standing Forward Bend. The most important thing is to listen to your body; pay attention to how you're feeling while you're practicing this dynamic folding posture. If at anytime you feel a strain in your legs or back, then you may want to modify the pose or even come out of the posture entirely. When it comes to yoga, with this pose or any pose, it is not necessary to push or force yourself into a position. It is about easing yourself into a yoga pose like this with lightness, safety, and integrity so you can gain the most out of each posture you practice.