To strengthen your lower back is to focus on your core, back and leg muscles. Quite often, the pain is associated with a muscle weakness in those areas. But to truly decrease your chances of aggravating your pain-prone lower back, you'll need to condition those muscles as well. And yes, there is a difference between exercises that strengthen and those that condition. You're either giving muscles more power (strengthen), or teaching them to move easily in a variety of different ways (conditioning). Even certain stretches are considered excellent muscle conditioners.
Again, strengthening exercises that reduce low back pain tend to focus on your core, back and leg muscles. Think planks, seated cable rows and squats. For conditioning exercises, think about the following
1) SUPINE KNEE TUCKS: Lie down on your back (supine position) with your knees bent and feet on the floor. Contract your abdominal muscles, pressing your lower back into the ground as you slowly bring one knee into your chest. Hold for five seconds, release and repeat with your other knee to complete one repetition. If, and only if, you're capable of mild rotation without pain—you can lift both knees and drop them from side to side for a few repetitions upon completion of the above, but keep your abs contracted throughout to maintain support.
2) CAT STRETCH: Face the floor on your hands and knees. Contract your abdominal muscles and slowly arch your back toward the ceiling. Hold for five seconds, relax, then sit back on your heels. Breathe deeply to feel a nice stretch, then return to the starting position and repeat.
3) PRONE HIP EXTENSIONS: Lie down on your stomach (prone position), resting your forehead on your hands. Contract your abs and glutes, then slowly lift one leg off the ground about six inches. Hold for five seconds, keeping your knee straight the entire time, lower and repeat with the other leg to complete one repetition.
4) HAMSTRING STRETCH: Tight hamstrings are quite often the biggest cause of low back pain. To keep them loose and limber, you'll want to constantly stretch them out. But the typical sit-and-reach hamstring stretch isn't always the best option for those who suffer from low back pain. I recommend you lie down in the supine position with legs extended. Bend one knee in so that it points toward the ceiling, grab behind your hamstrings and straighten your leg as much as you possibly can. Hold for 30 seconds, lower and repeat. Then switch legs.
Of course, this list is incomplete. There are so many ways you can strengthen and condition a weak lower back. Feel free to contact me for additional ideas, or leave a comment to share your favorite with my readers and myself.
Remember: Seek help from your doctor if your low back pain is persistent. It might be something more than a muscle weakness or imbalance.