Yoga can be tricky with osteoporosis, a bone thinning disease of aging, because osteoporosis is associated with spontaneous small fractures of the spine, hip and wrist.  For hip flexibility, try seated postures like the Lotus and hip-openers like the Goddess.  Poses like the Cow-Cat stretch are good for the back, and standing poses that promote balance are protective against falls. Before you start, get a doctor's clearance and always use caution when moving in and out of poses. You also need to build up your muscles to support your weight during the poses or stick with beginner classes that are adapted to your level of strength.  There are several fine books available on the topic of yoga and osteoporosis, and you can read about yoga for seniors at About.com.