Yoga is good, but you have to be very careful. My friend took yoga classes with an experienced instructor, and even then, she ended up hurting her back a LOT because of it. She never had back problems before. One of my friend's father, who is hindu, practises yoga and is also a doctor, actually does not recommend yoga for everybody. He says it's better to have your back checked by a professional before doing it. At the very least, stick to the simple positions and don't force anything, especially if you're doing it alone. And if you take classes, make sure there aren't too many students in the class, so that the instructor can correct you right away if you're doing someting wrong.