In seated postures like the hamstring stretch shown above, yoga straps can be used around the feet as shown. Arms should be straight, and grab hold wherever you can. The pelvis will naturally tilt forward, bringing the body into alignment and bringing length into the hamstrings. It isn't often that we sit like this, so shoulders might creep up toward the ears with tension. Be mindful of that, and relax the shoulders away from the ears, keeping the spine long and the shoulder blades melting down the back. Sternum is lifted.