One way to get better at yoga asanas is to imagine it. While I'm not advocating pushing too hard, sometimes just imagining you can go further into a pose will get you there. It gives you the will to try just a little harder. For example, if you're doing Bow, visualize the beauty of the pose: the graceful arc formed by the arch of the back, the legs lifted high, feet moving back to open the chest. Then make your body into that shape—not violently or suddenly, but using your intelligence, your breath, and gradual movements from the inside to achieve the best pose possible.
So next time you're doing a pose you find difficult, instead of doing it halfheartedly, thinking, "I can't do this pose," try this little trick of mine. Say to yourself, "I can do it." Use your mind's eye to see yourself a little deeper in the pose and then go there.
By the way, this will also help you hold a pose longer. Summon up your willpower, take deep, even breaths, and see yourself holding it longer, a second at a time. You'll be pleasantly surprised when you see just how well you can do!