I agree with Maheswari and HathaTeacher. First, it's not about getting the pose perfect; it's about awareness. You may be surprised to know that it doesn't actually matter how far you can stretch. It is how the pose makes you FEEL inside. Extend your awareness into that tight muscle; don't fight it, just feel it. Secondly, the breath is the key. In HOY, the system that I teach, although not in AYP, we use ujjayi breath along with the asanas. But even if you are not using ujjayi breath, it is crucial to let the breath guide the movement. Think of it as the breath moving the body. On reaching up or back, we inhale deeply. On bending forward we exhale completely (and in more advance work the bandhas come into play). We do not push the asana beyond what the breath will comfortably allow; if the breath becomes short or ragged, you've gone too far. Continue to breathe deeply and relax into the posture. Think of breathing into the tight muscles. When your awareness is really focused this becomes incredibly pleasurable, even ecstatic; asana becomes a form of meditation (or depending on your orientation, prayer) with the whole body. And that is the whole point - not whether or not you can reach your foot. As you continue to practice you will indeed become more flexible. But don't look at it as a "goal" to be reached; rather, ENJOY your practice in the moment! (of course the other suggestions on diet, herbs, etc. are also useful)
