Updated June 30, 2014.

Tight hamstrings, which can be exacerbated by spending a lot of time sitting, can lead to back pain and other discomforts. Runners and other athletes can also benefit from incorporating hamstring stretches into their training. Improving your hamstring flexibility is usually a slow process, but it's possible with regular practice. 

Beginners

Head to Knee Pose - Janu Sirsasana
This seated stretch allows you to focus on one leg at a time, which is a nice alternative to a classoc forward bend.

 Try to keep your extended leg active by flexing that foot the whole time. 

Pyramid Pose - Parsvottonasana
In standing forward bends like this one, make sure you are not hyper extending in your knee. Even though you leg might not look as straight, a microbend at the knee is a safer position for your joint health. 

Reclined Big Toe Pose - Supta Padangustasana
A strap is going to be a really useful prop for anyone with tight hamstrings. Using the strap to close the distance between your hand and your foot in this pose, for instance, allows you you to straighten your legs and get the full benefits of the stretch. If you don't have an official yoga strap, don't worry. Any belt or scarf will work just as well here.

Seated Forward Bend - Paschimottanasana
To get the most out of this hamstring stretch, it's important to keep the legs engaged and the spine long as you come down. Imagining the pelvis as a bowl slowly tipping forward helps encourage the correct rotation of the torso over the legs. 

Seated Wide Legged Straddle - Upavistha Konasana
For people with hamstrings, just sitting in upavistha konasana is a big challenge, never mind coming into a forward bend.

One of the best ways to find some space in this pose is to bring some lift with one or more folded blankets under your seat.

Standing Forward Bend - Uttanasana
Here's a technique you can use with any forward bend. On your inhales, come a little bit out of the pose and lengthen your spine. On your exhales, forward bend. In uttanasana, you can bend you knees a little if it feels better than straight legs.

Triangle Pose - Utthita Trikonasana
As in pyramid pose (above), make sure not to lock your knees in triangle. You can rest your hand on your ankle, shin, the floor, or a block. Pick the one that allows you to really open your chest to the side.

Standing Straddle Forward Bend - Prasarita Padottanasana
A common mistake in this pose is to take the legs too far apart in an effort to get your head close to the floor. Keeping the legs at about a 90 degree angle allows for a good hamstring stretch and it's a safer position for the hip joints.

Intermediate

Half Moon Pose - Ardha Chandrasana
Taking time to establish good alignment in your legs before lifting your arm. Having your hips stacked insures that you can open your chest to its fullest potential. Use a block under your hand if necessary.

Revolved Triangle Pose – Parivrtta Trikonasana
This pose is often very challenging, even for experienced yoga students. A block under your hand and/or taking the hand to the inside of your front foot are both good options to keep your hips from going all cockeyed. 

Sleeping Vishnu Pose – Anantasana
This pose always looks easier than it is. If you keep the side of your body very straight, balancing becomes a real challenge. Use a strap around your foot if you can't reach your toe with the leg straight.

Advanced

Heron Pose - Krounchasana
You may be able to get into this pose more easily if you are rocked back on the sit bones, but this will cause your spine to slump. For the correct alignment be on the top of the sit bones with the spine straight and keep your leg slightly bent if necessary

Full Side Plank - Vasisthasana
Adding anatasana (above) to your side plank gives you full vasisthasana. As with all these advanced postures, proceed with caution. It takes time to get all the necessary elements to come together for a pose like this.

Monkey Pose - Hanumanasana
The ultimate pose for hamstrings? The splits, of course. Again, use props liberally as you work on these poses. Blocks under your hands are a good place to start. As you get closer to the floor, a block under your front thigh can be stabilizing. Be careful coming out of the pose and have patience!