+1 on stretching hamstrings and hip flexors (good to do that all the time, anyway). When those muscles are tight they can start pulling on the muscles in your lower back.

Just to use a slightly related example, I was recently getting some really bad chronic headaches in the back of my head. A chiropractor took a look and it turned out that my neck muscles were really tight and were pulling on the muscles in the back of my skull, causing the pain. Doing some neck-stretching exercises on my own and putting my neck in traction at the chiropractor's office has helped eliminate the headaches. Just goes to show that a lot of aches can be caused caused by neighboring muscle groups that are too tight.