I'm considering spraying a tight raw pair I have with water from a spray bottle and then wearing them outside during active hours. I'll let you know how it turns out. Wearing them while you sleep might help too.
I had a pair of particularly non-stretchable jeans that I really needed to loosen up. I just wore them and did everything in them. Lots of stretching, some running, went for a really long bike ride (which was difficult!) ert etc. My efforts paid off.