A teacher of mine took us through a great exercise in boat pose the other day. Start the pose with the knees bent, shins parallel to the floor, and hold onto the backs of your thighs so that the spine and legs are making a nice V shape. If you can maintain the V, let go of the thighs. Stay here or extend the legs straight, but only if you are keeping the thighs and spine in the same V shape. If you progress along and discover that your legs and torso are getting further and further apart, go back to one of the preceding versions.