The following 7 minute video will teach you some basic yoga moves for strengthening and toning the core. I use this sequence in my classes all the time and my students have reported that it has helped diminish their back pain. I recommend doing this once you are already warmed up. A few sun salutations beforehand will really get your blood flowing and make the sequence work better. Afterward, do a few stretches like cat/cow and cobra pose to lengthen the front of the body.