Hip flexor stretches -- stretches that target the psoas and iliacus muscles -- are essential for eliminating lower back pain.
Because the psoas attaches along the lumbar spine, tightness in this muscle can cause an excessive lordotic curve which can trigger the lower back muscles to also tighten.
Tightness in the iliacus, on the other hand, causes a different type of discomfort. Due to its location tucked in the bowl of the pelvis, iliacus tightness can be responsible for a range of symptoms including sacroiliac joint pain and hip pain.
The stretching method I most frequently advocate is a repeated 2-second stretch (commonly known as Active Isolated Stretching) as opposed to static stretching.
I prefer this method because it does not provoke the protective stretch reflex, a reflex in which the muscle actually braces against the stretch.
If you've ever attempted to stretch a muscle, held it in a static and strong stretch for a period of time, felt the muscle start to shake, and you were unsure if it was actually lengthening... You may well have experienced the protective stretch reflex.