Okay, so if your lower back is too curved, it could be the opposite- lordotic, which is a very different set of stretches and exercises. Lordotic would mean you have weak hamstrings instead of tight ones. In any case, here is a really good upper hamstring stretch. Put your leg on a chair and make sure you are facing forward and your back foot is straight. Now, bend the knee of the leg that is on the chair and keep your back straight as you gently bend forward. This is a great upper hamstring stretch that should hit that area.