The secret to true and lasting flexibility, freedom of movement lays in TONING opposing muscles you are trying to stretch. Meet PNF or active stretching.
If you do VASIE Pilates you experience this type of stretching on a regular basis. Studies show this type of stretching is more effective. [read NIH study]
The basic premise is that you are using one set of muscles (let's say your abs) to stretch out the muscles on the opposite side of the joint (in this case - back muscles).