This article is from the Bicycles FAQ, by Mike Iglesias with numerous contributions by others.
Another key to preventing low back pain is to keep your abdominal muscles
strong. These muscles help support the back. Do abdominal crunchers,
not sit ups. Sit ups emphasize the hip flexors, not the abs, and can be
hard on the back. Crunchers are done by lying on your back with your
knees bent. Press your low back into the floor and curl your head and
shoulders off the floor. Hold for a couple of seconds, then lower back
to the floor. Repeat until you can't get your shoulder blades off the
floor. Abs can be worked every day.
Strengthening the low back muscles can also be helpful. To start, lie on
your front with your arms and legs extended in a straight line with your
body. Raise your right arm and left leg. Put them down and raise your
left arm and right leg. Put them down and continue. As your back
strength improves, try raising both arms and legs at the same time, arching
your back in a "reverse stomach crunch". There are, of course, more
powerful back exercises, but they are also more stressful and shouldn't
be considered until your back is 110%.