The muscles in your back as well as the spine itself need to be stretched if you want to help relieve lower back pain.

Here is a great resource with explanations on which lower back stretches to do and how to do them:
Lower Back Stretches at PhysioAdvisor.com

The site is a physio site so they do have a good idea of what they are talking about. There are also photos of each stretch, which really helps you figure out what yuo are meant to do for those of you doing these at home.

Some of the stretches include:

• Lying Rotation – the one where you move your knees from side to side, nice and gentle for your back
• Prone on Elbows -  lie on your back and prop your self up on your elbows, this mildly stetches the spine
• Knees to chest – helps stretch and realign your back
• Standing Side Flexion – stand and move your hips side to side, great for loosening the lower back
• Lying Extension – same as the prone on elbows except prop yourself up to your hands
• Standing Extension – stand and arch your back backwards, natural move to stetch the back
• Standing Side Bend – like the standing side flexion but you use a wall to support you, allows you to stretch more without falling over
• Some leg stretches that also gently stretch your back

These stretches are ordered from easiest to hardest, so start off doing a lot of the easy ones every day and then move on.