My flexibility has not improved either and I don't do static stretching anymore because of that reason. I do PNF now and my flexibility is actually improving. I use either my own bodyweight, the wall, or the floor. I use some object to put all my weight on for a certain muscle for the isometric stretching part. For the passive stretching part, I would do the same kind of stretch pose I would do for static. I do passive stretching for 15 sec followed by isometric stretching for 15 seconds, followed by passive stretching again for 15 seconds for one muscle. Then I go to the next muscle. Example - I do the glute stretch, then I do the pigion (all I do is flip over), then I do the glute stretch again. Then I switch legs.
What may work for me may not work for you.
PNF stretching - Wikipedia, the free encyclopedia are all the different options on PNF. There are 5 ways you can do PNF. For exact directions on how to do it, go there.