While often known for its ab-busting potential, boat pose (known in yoga as Navasana) also works the deep hip flexors, as well as the spine. When you add in the arms, even your shoulders will get stronger.
How to do it: Start seated with your legs extended in front of you. Press your hands into the floor just behind the hips, pointing your fingers forward. Lean back slightly and lift up through your chest, to keep your back from rounding. As you exhale, bend your knees and lift your feet off the floor until your thighs are at a 45-degree angle from the floor. Straighten your legs slowly. When you feel stable, lift your arms off the floor and bring them out in front of you, parallel to the floor with the palms facing each other. To come out of the pose, lower your legs and arms as you exhale.
How to get better: If your hamstrings are tight, keep the knees bent so you can maintain the neutral shape of the spine—similar to as if you were sitting in a chair. For a more intense workout, lift your arms overhead.