As a physical therapist who spends his days teaching people how to stretch to help their backs, if someone was to ask me this question, my response to them would be to ask them what part of the back to they want to stretch. If the answer was to stretch their lower back, my response would be “Don’t Do It!” So now you’re probably thinking why not? Let me explain. Parts of the body function in one of two ways, to be mobile or to be stable, but not in both ways. The parts of the body (or joints, or sometimes a collection of joints) alternate as you move from one to the next. For example, the ankle, which can move quite a lot, tends to be a mobile joint, while the knee, which really only has motion in one plane, tends to be more of a stable joint. The hip is more of a mobile joint, while the low back, or lumbar spine, tends to be more for stability again. Above that, the middle back, or thoracic spine tends to be more of a mobile collection of joints.
By stretching, we are attempting to make a joint more mobile. Remember, a joint can either be mobile or stable, but not both. By attempting to stretch the lumbar spine, we are trying to make it more mobile, which takes away from it’s function of stability. So if you have a bad back, and are trying to help the back, stretching the lumbar spine is actually going to make it worse! To help out a bad back, what we want to do is to try to make the lumbar spine more stable or to make the joints that surround the lumbar spine, the hips and the thoracic spine, more mobile. If a body part that is suppose to be mobile isn’t, then the body attempts to get that mobility somewhere else, usually the next body part in the line. So in other words, if your hips aren’t mobile, or flexible, then the body will try to make the lumbar spine become more mobile, which will take away from the chief function of the lumbar spine, stability.
With that in mind, if we want to stretch to help our back, we need to focus on stretching for the hips and thoracic spine. There are a lot of muscles that work at the hips, which is one of the reasons the hips are one of the most mobile joints in the body. Some of the muscles to focus on are the hip flexors (the psoas major and the iliacus), the piriformis, the hamstrings, and the rotators of the hips, both the internal and external rotators.
As far as stretching guidelines go, first of all, stretching cold muscles won’t get your very far. It’s like trying to bend cold steel. If you want to bend steel, you have to add heat to it. Same thing applies to our muscles. If we want to lengthen the muscles (which really is what we are doing when we try to stretch a muscle), we have to heat them up too. The way to heat muscles is to either add heat directly, or to actually just use them, which is what I suggest. Cardiovascular exercise works best here, like biking, jogging or even walking, for a minimum of 5 minutes. Also to change the length of the muscle, you need to hold the stretch for a minimum of 15 seconds. In the clinic I have my patients maintain the stretch for 30 seconds.
Hope this helps. You can find lots of good stretching instruction on the internet, and even video demonstrations on websites like youtube.com.