There are many exercises to strengthen your lower back, first don't forget about your abs. Having strong abs can prevent back pain and having a strong back can help your abdominals. Pilates is a really good exercise for your back. Try supermans, were you lay on your stomach on the floor with your arms straight out in front of you like your flying. At the same time raise your head arms and legs off the floor, you can do ten of these in a row or hold one for 30 seconds. Another one that is really important is balancing on one leg. Hold one leg out in front of you while you squat with the leg you are standing on. As long as you keep your abs tight and your core engaged during your exercise anything will help your back get strong.