This site has some great photos and explanations for lower back exercises.

Strengthening is important in order to have muscles that support your back properly.

Let us have a look at what is covered in the strengthening exercises:

1. Transversus abdominus retraining – this has a fancy name but is basically lying down and thrusting the pelvis.  This is a nice and easy exercises.
2. Opposite arm leg raises – you lie on your stomach and raise your opposite arm to your leg.  This helps strengthen your back without putting any stress or physical pressure on it.
3. Prone hold – This is also known as the plank.  It helps strengthen your abs and lower back.  Most people can only hold it under a minute, but be careful not to push yourself too far as otherwise you will have saw abs for day.
4. Supermans – you lie on your stomach and raise your arms and legs.  Has the same effect as number 2.
5. Side holds – also known as side planks.  Strengthens different muscles in your lower back then the frontal plank, which strengthen more of the back supporting muscles.
6. Resistance band rotations – add some twists with resistance.  Just make sure to do a stretch and warm up before you do this.

There are also explanations on how to do further and harder versions of these exercises using a swiss ball.  This is a great investment if you are quite a fit person and specifically need to work on strengthening for back pain.  They also allow you to do a lot of different stretches on them, so are a great investment anyway.